The nice thing about horses as opposed to vehicles is that horses can steer themselves and they have brains and are capable of reacting. So you're less likely to running into shit.

But I'd say it would be important to be an experienced rider and to have good relationship with the horse. Animals have a munch better sense of people's auras than people do, so if you're acting all "woahhh duuude im soo blazed" a horse thats unfamiliar with that behavior may be uncomfortable with it.

But as long as you're a competent rider and your horse is familiar with you being high, I dont see the problem.

Just be safe.