After the debacle that was My First Pet, they're going to be really leery about a pack that ties too closely into another pack. The "Pet Show" idea is a bit too tied in with C&D to add to a pack with horses.
My theory is that we get a farming/rural pack, and it features a horse ranch lot (especialy now that we have the 64x64 lots) where you board and train your horse rather than having it at home. With the loading screens, riding a horse from home really isn't all that practical, but a horse ranch would have the space to have all sorts of training areas for racing and jumping. Not sure if we'd get a full-fledged Create A Horse but they could have a choice of breeds; one might be better at racing while another better for jumping or tricks.