I'd love to see horses. I'd definitely be interested. In my case, I wouldn't actually need to have wild, roaming horses in the game unless a world were created to suit them. I could see a world that combines elements of Australia and Colorado or similar, having some wild horses. I'm not personally looking for the wild horses but I'd enjoy them even primarily in one world or I'd keep them on very large lots. I still haven't used the biggest lot in Newcrest, for example.
I'd be mainly interested in the equestrian skills and doing some horse chores. I liked the TS3 approach to horses a lot. I'd be happy to see some forms of that again - racing, horse shows, jumping, rodeo, etc. I don't know, I'd just want lots of things to do with the horses and I'd have sims who would ride and train them. Anyway, I'm the market for this.