but humans are not crouched over the horse as if they are about to snap it's neck in half....
Lions and tigers are wild animals...they are unpredictable, no matter the extent of the training.
If the horse was not scared, you would not see the whites of his eyes, and his head would be lower - he is trying to avoid his ears touching the lion...he's scared.
It's not impossible to perform, because as we all have seen it has been done - the question is to weather it should be done or not...and my vote says "no".
This is no different than asking a person with a papercut to swim in front of a "trained" great white shark.