Monster-Slaying Daemonhunter
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I have a headcannon to explain the ordering of Imperial vehicle development and deployment. You see, all the different model version do have official marks and patterns, indicating that a MkIIc Rhino is in fact a different model from a MkIb Rhino and so on and not just "the model was updated, and now it's better looking".
So, we have, in the Rhino family:
MkIB Rhino
MkIC "Deimos" Rhino
MkIIC Rhino - Current Rhino kit
old Sisters of Battle Rhino
1st Edition Predator
MkIIIC Predator - Notably featured a number of glacis options
"Deimos" Predator
MkIVB Predator - Current Predator kit
MkIVC Predator
old Vindicator
"Deimos" Vindicator
current Vindicator
MkII "Mars" Whirlwind
MkIIB Whirlwind - Current Whirlwind kit
"Helios" Whirlwind
"Scorpio" Whirlwind - Is notably meaningfully different from the other Whirlwinds and a variant instead of a mark, but there currently isn't to my knowledge a current model for a regular Whirlwind on a Deimos chassis
Hunter
Stalker
Repressor
MkI "Orphelia" Immolator
MkIIc "Sanctorum" Immolator
MkIIIa Immolator
"Deimos" Immolator - Current Immolator kit
"Piroris" Exorcist
"Sanctorum" Exorcist
"Deimos" Exorcist - Current Exorcist kit
old Razorback
current Razorback
Key changes between these various models are the glacis, the shape of the tracks, and the shape of the doors
The fact that the Immolator and Razorback were developed in M36 after the proto-sisters discovered the partial diagrams for the former make the situation more complicated, since that means that the versions on pre-MkIIc Rhino chassis must have also been produced after those chassis marks were discontinued.
In addition, the Predator Annihilator [twin lascannons] was only developed in M36, unrelated to the Razorback and Immolator programs, but twinlas variants exist of the Deimos and MkIIIC Predators, so these also must be back-retrofits.
So, without further ado, my Headcannon:
Founding
At the founding of the Space Marine legions, it was apparent that they would need a fast and easy to maintain APC to equip them, and the Rhino MkI was developed. After revisions to the initial design prototypes to better meet specifications, the Rhino was accepted into service as the MkIb Rhino and placed into mass production.
It was desired to have as much of the legions' armouries be of a family, and so the MkIIIa Predator [First Edition Predator] was developed and placed into service. It was named the MkIII, as it replaced previous unrelated FSV's in pre-Imperial service that shared the same name.
Similarly, in order to provide responsive artillery support to the legions, the MkII "Mars" Whirlwind was also developed, replacing previous rocket artillery platforms in pre-Imperial service.
The Great Crusade
As the Great Crusade wound up, the MkIIIa Predator was discovered to have a multitude of flaws and production bottlenecks owing to it's interior layout, the design of its sponsons, and the architecture of its turret. Thus, a program of revisions was begun for it. The revisions redesigned the sponsons to be a simple "riveted box" construction and moved the turret to the front of the vehicle to improve access to the engine space and working conditions in the gun turret. The revised design also called for simplifying the domed turret into a riveted hex turret similar to the sponsons, but this revision was rolled out less universally across new production and many factories continued to use their existing casting equipment to produce domed turrets for their Predators. Revised Predators with the domed turret would be the MkIIIb, and fully revised Predators with the new hex turret would be the MkIIIc.
The lessons of the early Great Crusade also demonstrated the need for a artillery vehicle to clear strongpoints, and engineers created for the Legion the MkIa Vindicator mounting the Demolisher cannon on a MkIb Rhino chassis.
The Deimos Program
As the great crusade wore on, it became apparent that the MkIb Rhino chassis did not have fully adequate mobility in bad terrain, and the broad "Deimos" modification program was initiated to address this deficiency across all vehicles of the family. The program added an extra road wheel to improve ground pressure distribution across the track, and also added raised idlers to improve obstacle and trench crossing ability up from nearly none. This change was accepted for all vehicles and entered full scale production; fully replacing the previous vehicles by the time of the Horus Heresy.
The Predator had always proven problematic, and even after it's first revision there was general dissatisfaction with its performance. The sponsons were large and heavy, the turret cramped, the forward placement of the turret overloaded the front road wheels, and overall performance of the vehicle was generally poor. The small hex turret also limited upgradability, and a number of new weapon systems were becoming available that were wished to be able to be mounted to the FSV. The Deimos program saw severe changes to the Predator design, with armor for the sponsons being dropped entirely, and the turret greatly enlarged. The turret returned to the dome shape to save weight due to its increased size, and was moved back to the rear of the vehicle to preserve the weight distribution of the vehicle. The large turret allowed for considerable upgrade potential, and variants were created with plasma cannons, conversion beamer, and flame guns.
the Breaking of the Legions
The widespread destruction of the Horus Heresy across the Imperium meant that the foundaries that supplied the Space Marines would have to be rebuilt virtually from scratch. With the TO&E of the Space Marines changing and the loss of existing founding equipment, it was simultaneously decided to completely redesign the Rhino and its family of vehicles. The new track design was considered successful, but a number of other changes were desired. The circular split-door hatches were slow to exit through and required some degree of contortion to exit from. In addition, The earlier hatch designed provided severely limited visibility and mounting guns on top made them heavy to open and nearly impossible to ride unbuttoned, making visibility exceptionally poor. A number of designs under the MkII program were proposed to meet these new requirements, and the sum of their changes after revision would be adopted as the MkIIc Rhino. The MkIIc Rhino featured rectangular drop-ramps for fast and easy loading and unloading, and replaced the hatches with large cupolas with periscopes and split hatches that drastically improved visibility from the vehicle. In addition, the multipart riveted glacis was redesigned into a single-piece glacis with a armored protector over the vision blocks, which was easier to produce and offered better protection.
The Deimos predator has been generally successful, and the unarmored sponsons were retained for the new Predator. However the turret would be redesigned again for a large, welded hex turret with sloped sides which would be easier to produce and provide better protection than the domed turret, at the expense of increased weight. Unnecessary armament variants were dropped. The glacis plate was also revised, using the same plate at the new rhino instead of the former "bulged" design. The turret was moved forward in the hull to a more central position to make space for an improved power pack at the rear of the vehicle. This design would undergo several revisions before final acceptance as the MkIVc Predator.
Several new vehicles were also developed at this time, including the Stalker and Hunter AA tanks; which were proposed as competing designs to meet the needs of the new Chapters in air defense but were both accepted into full production.
A dedicated tank destroyer platform was also proposed as a Whirlwind variant carrying Top-Attack Guided HEAT missiles, but was deemed superfluous to requirements of the Space Marine Chapters and rejected for service. [this would be the "Sanctorum" Exorcist]
The Sisters of Battle in the Age of Apostasy
The expanding Ecclesiarchal forces also discovered the Immolator diagrams during a crusade, and engaged with negotiations with the Mechanicus in which the AdMech would gain the knowledge and the Eccelsiarchy would gain exclusive rights to the product of the cache. The AdMech began development based on the designed recovered, and combining them with the existing MkIIc Rhino produced the Immolator tank. The new IFV featured a raised superstructure to accommodate the traversing gear and turret basket, which also made its way as a new MkIII Rhino variant which was slightly larger and featured better vision ports for the passengers. The MkIII Rhino did not find widespread acceptance even within the Ecclesiarchy due to the prevalence of the MkIIc, but the MkIIIa Immolator, which was initially considered a "Armed Rhino" before being named "Immolator" by it's recipients, did.
The foundation of the Brides of the Emperor under Vandire saw the next major additions to the Rhino family. The newly created organization needed equipment to turn it from a small conclave of fanatics into the powerful army of enforcers he envisioned. They would be equipped with Rhinos as their APCs, but they needed a tank of their own to give them peer parity with the other military branches of the Imperium. For this, they turned to the discarded tank destroyer design proposed for the Space Marines after the breaking of the Legions. Some revision to use vertical launch cells instead of launch-at-angle boxes and extensive decoration later, it entered into production as the "Prioris" Exorcist.
After the Age of Apostasy, the fall of Vandire, and the officiallization of the Brides of the Emperor as the Daughters of the Emperor, the Adepta Sororitas, a massive expansion of the new organization began on a scale not seen since the founding of the legions, taking them from a single moderately large group of fanatical enforcers of 6 very large fighting orders. To accompany this expansion, many tanks were required.
As an expediency, MkIb Rhinos were taken out of mothballs and assigned to the growing orders, and a conversion kit was developed to covert them into Immolators as a stop-gap to meet their needs.
In addition, the production of the more complicated MkIII Rhino chassis for the Prioris Exorcist and the Immolator was a bottleneck on production and considered superfluous by the AdMech. Production of Exorcists would be supplemented by using the original design spec and a redesign of the Immolator was conducted to eliminate the superstructure; both were accepted in large numbers as the "Sanctorum" pattern to meet needs. However, the Immolator turret imposed upon the driver's bay without the superstructure, and the angled launch from boxes was generally considered less effective at providing all-round strike capability than the vertical launch for the Exorcist [to make no mention of the lack of ornamentation that the church desired, though that would be an addressable problem].
In M36, the Space Marines unearthed another partial design for an armed Rhino variant. The widespread success of the Immolator in Sororitas service made similar capability highly desirable for the Space Marines, and the preliminary data was turned over. The turret on the "Sanctorum" Immolator design would be moved to the back of the troop bay, where it would impinge on the troops instead of the driver, and with automation technology from the Space Marines' data was also reduced in size. This vehicle would be accepted as the Razorback. In order to meet demand for this new vehicle which came at the same time as the AdMech was already working on meeting the demands of the Sororitas, many surplus MkIb's were taken out of storage and given a conversion kit and issued to the Space Marine Chapters.
Predator Annihilator
During M36, a field conversion of the MkIVb Predator using twin lascannons started making the rounds of the Space Marine chapters, and demand was brought to the AdMech. While the Lascannons fit in the existing hex turret, the AdMech further revised the turret design to straighten the lower faces of the turret armor to make more space for the twin "breeches" of the lascannons, titling it the MkIVc Predator. Demand for the new tank would be high after it's sanctioning due to the lack of a good tank destroyer for the chapters though, and given that it did work with the regular turret, production would initially begin with the normal MkIVb turret and the MkIVc would fall by the wayside.
Retrofit kits were also rolled out for the MkIII and "Deimos" Predators to help meet demand. The MkIIIc's hex turret couldn't accommodate the upgrade, so only the MkIIIb's domed turret could be converted.
The Modern Era
The AdMech had generally sought to eliminate manufacture of the MkIII Rhino and the two tanks that were dependent upon its production, and production of two types of Exorcist and Immolator as a response to low production rates of the MkIII chassis was an unnecessary strain on the supply chain and production infrastructure with all of the imperial military organizations expanding again in the wake of the 13th Black Crusade. The Sanctorum patterns, the AdMech's preferred solutions, were generally unpopular with the Orders, and more Priors production, the Orders preferred solution, wasn't possible for the AdMech to meet while also dedicating even more production space and time to Cawl's new Space Marine tanks. The superstructures were cut away, streamlining production. The new Immolator turret was drastically increased in size and the guns moved inside the protection of the turret. A new hull heavy bolter was introduced to a revised glacis. As a consequence of the massive weight increase of the turret, the need to preserve some access for maintainence, but the reduced need for use by troops, the Deimos patter doors were reintroduced to the frame. This was, in some way, the worst of all worlds, still requiring separate vehicle production lines, but the vehicles themselves to come off were generally considered better vehicles.
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"Hard pressed on my right. My centre is yielding. Impossible to manoeuvre. Situation excellent. I am attacking." - General Ferdinand Foch 
