I personally hope for Horses in Sims 4. Who's with me? And what might it be like?
Apart from that, I think horses WILL be in Sims 4, because it's another opportunity for EA to make money. I think, logically, they would show up coupled with some sort of "Pet Show" thing - for example you can take your Cats and Dogs to the pet show but of course you need both EPs. They work well together and deepen each other but don't require each other. It would also be nice to have a new farm-based world that would be great for roaming on your horses.
I'd hope they'd show in a GP but they'd require a lot of animating (like Cats&Dogs) so maybe not. Perhaps they'll be another EP with as much stuff crammed in as possible to make it 'worth it'. Perhaps that will be the pack that gives us minor pets that involve only "Pet" and "Feed" interactions.
"I'm not saying Stuff Packs thrill me to my wretched, rotting core. I'm just saying that we only get one expansion pack a year because Devs only have one pair of hands. You want to buy more half-finished packs like Get To Work? Do you?"