This has been discussed a lot previously. And the pros and cons. Simple answer, only EA knows.
And if they do bring horses in, if we wanna ride ém off to somewhere for fun its prolly gonna be like how cars were in The Sims 2 with how riding in ém, minus going to comunity lots, always had the car and Sim vanish before they return. Of course, in The Sims 4, they'd prolly have more room to move but still.
Will EA give us horses in The Sims 4? That's a question some would wanna know, huh?
Farming and Equitation were separate entries for voting on in a poll GrimSuruDoi put on Twitter some years' ago and both came high. We recently got farming (of sorts) in Country Living and it could be that Equitation could come as a game pack later, maybe with ranches or competitions of some sort.
It would be too much to have put all that in Country Living I expect but I would hope it might come with barns, stables, fields of hay and tractors at last. Sims 4 has a tendency to put a lot of stuff in earlier packs that really are more useful in later ones. We've got some of those, like bales of hay in Country Life and the off the grid objects in StrangerVille.
The game (hypothetically; not confirmed) has 10-20 more expanded years to live. Maxis is bound to lose ideas what concepts to implement, so I'm sure they'll dedicated their time to the demanded features like horses return, with or without open world (much like we got boats and bicycles).