Well, in my world they are.
And, by the way, there is no need to ride them ...
I rather see them as strong creatures with their own free will - very similar to what people usually think of Dragons.
In my own fantasy world, Unicorns are very similar in terms of life-spand and wisdom and powers to Dragons. With the only exception that Unicorns cannot fly and are therefore earth-bound.
In the multi-author-story "On the road again" of the German-language community here, I let one Unicorn drastically heal a horse, which is from then on travelling with it, because of something Star Wars fans would call a "life det", but in my invention it is rather some kind of IOU.
I wrote "drastically", because this horse needed very strong magic to be healed. In fact, part of the horse kind of consists or is constantly held together by pure magic the Unicorn used. At least, until the destroyed body is reasabled again. Wounds and bones need some time to heal naturally. Therefore, the body was held to gether by the Unicorn's strong magic, kind of a mixture between a broken arm in plaster and a kind of "cyborg" held together by magic (you can see a kind of opposite to that in Darth Sion of KOTOR2). But when all bones and the flesh is healed, the magic will disappear, because there is no need/use for it anymore. But until the body is fully healed, the horse is in part held together by the Unicorn's Magic.
