That would have to do with models and stuff anymore. You can't simply mount it in game like that. So you'll have to create a model of let's say a footman, and then a model of a footman riding a horse, and a horse. So when a horse loads a footman, remove the units and create the footman riding on a horse.
Hmm i like the Hippgryph one, but is there a way to make that able to mount multiple types of units besides making multiple of that spell? because i plan to have at least 4 units that can mount it right now, and probably more later.
how about this
you use on your mount the load ability [transporter idk]
then when the spell is used and the unit monted make an SFX with the model of your hero attachment point chest and put it in a var when the mount unload the hero destroy the SFX
ps i think it will show some blood and lay the death animation when you destroy the SFX