Horses do not get used to unfamiliar environments. They have an environment they like and anything else kinda stresses them out. They’re big dumb skittish animals that long for quiet open fields of grass with nice firm rocky soil.

So yeah, taking a horse and throwing it on an aircraft would spook the bajesus out of it. Throwing it off again would probably scare it to death, if the landing, however gentle, doesn’t kill it