A few things about different species: First, nice job of making the buffalo Ogryns! Secondly, is that equine on the right suppose to be a donkey or mule? If not, try to get them in another guardspony's picture. Maybe as a replacement for another abhuman species? Thirdly, wouldn't it make some sense to have a zebra in here, to reflect the more exotic nature of the catachan regiments? I thought the pony up front with the sword was one, but then I noticed the irregular pattern of his "stripes." Nope, just scars.
Well, if you want to stuff everything from the show, why not rely on multiracial factions? A buffalo could totally be a Dark Eldeer Grotesque-equivalent as it and Wracks can be made from almost any race if I remember well. Or maybe a Khornate daemon? Aren't they prone to violence? Gryphons could easily fit as Scourges.
I think buffalo are fitting since the Ogryn are some of the most extreme mutations tolerated by the Imperium. You can't breed buffalo with horses, and I don't think you'll see a human mixing it up with an Ogryn anytime soon. Sanity-X couldn't just make Ogryns Big-Mac sized ponies, since he's already doing that with the Space Mareines.
Ogryns aren't mutants. They are abhumans - naturally occurring subspecies of Man on high gravity worlds who are bigger than even Astartes. There are records of half-Ogryn offspring, but they're rare and I really don't want to know how they happen. Additionally, with every new edition, Ogryn are less ogre-like and more giant strongman types. What's to stop Sanity from making Pony Ogryns like really big versions of those two slow colts from school?