I personally think horses wouldn't fit in at all. Both lions and wolves are social carnivores. While horses do herd, the last time I checked they didn't hunt. I don't see it working at all. On top of that, I don't hear very much on the concept of wild horses as I do with wolves and lions.
Actually, an equine addition would take the main thing out of the game. There's now, cat RP's and if you would add a equine their would be alot horse RP's. What I'm trying to state is, if their is a equine model their will be more people on horses then on Feline's and Canines.
I don't think that. Horses may be popular, but at the end of the day it would be either wolves or warrior cats that take majority. I don't like horses at all and would likely never play as one. Also, just looking in the forum RP, it's wolf RP, wolf life RP, wolf adventure RP, Another wolf RP, some pack life RP. I don't see much for horse RP, and have never considered horse roleplay popular.
In game I've seen more dragon roleplays.