Traversing the map can take a long time especially if you live far away from the monuments. So the addition of rideable horses would save most people alot of time.
You could find saddles in crates or barrels but they could be non-researchable. You could tame them with a growable crops like wheat, or you could find apples in trees and you would have to wait for them to fall off.
You would have to feed them or let them eat the grass. Wolves would kill them when you are offline if you dont make a fenced off area.