Alright....time to get some goats and sheep! I have a dog, and the horse does interact with the dog sometimes. But the dog is another animal fighting for my Sim's attention so sometimes social interactions from both of them stack up in my queue. The horse doesn't interact with the cow at all, and the chickens jump in fear every time the horse enters the area with the chicken coop! I'd like a second horse, but I've been afraid that both horses would just annoy the crap out of me.
If this doesn't work, I'll resort to locking the horse up behind a fence. I'd hate to do that because I like to let it run around the property, but it would solve the problem. And also prevent my horse from walking into my house. That's getting on my nerves too. I would just lock the horse out of the house, but I have it set to lock for all except family members so I can keep the pesky neighbors from randomly walking in uninvited.