I don't know how common place this is across homeworlds nor that which has been conquered, but I've recently learned that some of our brothers on fedual worlds are doing things the traditional way. Particularly when resources and supply chains are next to non-existent. To be specific the chapter militia, however as recruits are inducted into these militias some are clad like Astartes. The armor may be powered or not, but it is much smaller and lighter regardless. They do not compare in size or weight of a first born and it is a questionable practice to mimic presence of a battle brother. No... it's an offense.
Yet rather than to police that on distant worlds with little interaction other than their due tithe we may for now ignore it. The humans are equipped well enough to handle what they have to deal with. Though we will want to discuss transitioning this practice to something that distinqushes men-at-arms, base human "knights" from Adeptus Astartes marines, and lay ground rules with their Artificers.
For such a breed of horse to carry a first born it would need to reach five thousand lbs in order to bear the weight of a fully armored space marine, and as well any barding worn by the horse. This is unrealistic even with genetic engineering without the hand of the Emperor. The Templar regents on these low tech worlds cut off from supply lines have made an executive descision to use base humans in place of Astartes (marines they have no means to produce) to maintain their lands while only a single ancient remains to manage the chapter keep.
Here are a few images we captured (sorry embed appears not to work for these)
https://grok.com/imagine/post/0fbaa60b-9a76-4873-95c1-c44bbbd05814
https://grok.com/imagine/post/0e272995-9d26-4456-b078-7285ef1e8dec
https://grok.com/imagine/post/cf1054c1-c50a-461f-aa7f-b4582826b609