Barry M. answered 07/26/21
Experienced & Patient Tutor in Adobe Creative Suite & Studio Art
Animal trainers for movies, whether horses or other animals, look for things that the animals do naturally. A horse rearing up on it's hind legs, and laying down are both things that horses do naturally, just usually not together. The trainers work with some animals and identify ones that seem to do certain things more easily, then train them to "do them on command." They practice a lot, and reward the horses (or other animals) with treats.
The triggers for certain actions can vary from Pavlovian responses, certain words/phrases, hand or body signals, or a more direct action like pulling up on the reins, or digging in heals in the case of horses.
I hope this answers your question, if you even still use Wyzant, this question was pretty old - I'm surprised someone hadn't already answered it.