A woman was once eager to show me a trick that she had taught her stallion. She went towards the horse, raised her arms, and the horse rose on his hind legs. She looked at me for approval. When I gave none, she asked what was wrong. The trick is to get him to keep all four feet on the ground.
Any fool can get a stallion to rear in a couple of minutes.
This encourages a stallion to dominate. It takes considerable training to keep them under control. This is also true with parrots. Anyone can teach a bird to run up to the shoulder because a parrot will always attempt to scramble to the highest point. When you allow your parrot to run up to your shoulder, you are actually encouraging him to become aggressive.