So I was in AAA Hobbies buying primer and I saw a pile of rubber dinosaur toys. I figured I'd check and see if they had anything suitable for a hydra. Instead, I found a good-sized Green Dragon:
I magnetized the neck so I can change riders (in case I make a Supreme Sorceress rider later):
As you can see it's pretty large. I mounted it on a 90mm x 120mm base. One quick-and-dirty paintjob later and this was the result:
The rider is the Cold One Knight champion, but with Glade Rider legs.
Here's the manufacturer's website for the toy:
http://www.shopatron.com/products/productdetail/part_number=60445/740.0.1.1
Not exactly Golden Demon quality but it beats spending $57 on Malekith or $45 for Rakarth.
Apparently they make a "Four-Headed Orange Dragon" but it's far too large
IMO to use as a hydra, so my search continues. Still, this has got to be one of the fastest, cheapest, and easiest ways to add a decent-looking 550pt lord to your army.