Well, this idea has two sides. One is just having other animals, like wolves and eagles, added to the RPG. The other is more complex.
So, has anyone read "The Golden Compass" or seen the website for the movie? Well, if you have, and even if you haven't, each human has a "daemon" (demon). It's an animal that embodies the personality/spirit of the person. For example, if a person had a dog for a daemon, then their personality would be like a dog's: loyal and such. So what I'm saying is: why not horses?
Now, in the movie, the childrens' daemons can change animals as they grow, but the ones here don't have to do that. They can be an animal that was born at the same time as the horse, or one that the horse just found. A use for them could be if the horse falls in battle. Then the companion can go and bring the message to the horse's mate or herd that he/she is down, and, if already dead, their final words. Then they could either die when the horse dies and they have delivered their message, or continue to live. We could think of a cool name for them, too.
You like?
Eh, to me it simply sounds like a hrose having just a simple animal friend. Which we never said we couldn't have or anything, it's a choice, really, whether or not someone wants to rp them...
I know. They didn't have to have one, just an optional thing. Maybe just other animals could be added and just not have any special animal friends.