As said above it would be impossible at the moment to add a new model, but if it was, I don't think a horse model would be added. With the feline model, you can make lions, tigers, cats, bobcats, etc., and with the canine model you can make wolves, foxes, coyotes, dogs, and more. A horse model wouldn't fit in very well, there's not that much you could make.
If a new model would be added, I think just a hoofed (hooved?) animal in general would be the best way to go. Because then you could make horses, goats (I want goats in the game so bad ;-;), moose, deer, pig, sheep, and many more, instead of just being able to make a horse.
I hope you see what I mean, just a horse model wouldn't let you make as much as the canine and feline models allow you to make. That's just my opinion on this.