Horses are strategic resources, just like iron and copper and all the others, thus I believe it makes sense to require a tech to reveal them. Not like animal husbandry is useless, even if there are no 'animals' nearby, chances are there will be soon and you will want pastures. In civ 3 you required the wheel to see them IIRC.
Granted, I can see the logic in the argument somewhat over other animals and resources, however, it would logically make sense for a tribal/ancient society to know about different wild food types, like corn, rice and wheat. Also, they would have eaten other animals like roaming sheep and bovines, but once you research agriculture or animal husbandry, then you get more use out of those said resources by stabling them and breeding them and having a flock nearby as opposed to roaming around in a hunting party. Likewise, with the development of nets and other fishing implements, the yield , as it were would also increase.