Instead of making the head/butt meet the ball in the middle (which looks less like a bounce and more like a touch), try the following:
• make the contact occur about 2/3 of the way down instead of in the middle
• delay the body’s upward movement until right before the ball makes contact
• increase the amount the ball flattens before launching upwards, have it pop back into shape a few frames after leaving the body (it should also be in contact with the body a little longer while it flattens on the way up)
• consider making the “wind up”/ducking part of the movement happen as the ball is falling back down instead of on the way up
• add a pause after the pony’s body reaches the top of its movement, before it starts to go back down (you can have it move down very slightly for a frame or two after the hit, think of it relaxing after impact before getting ready for the next one)

You don’t have to add all of this at once, and some of it depends on the frame rate, but I just wanted to add a couple tips about making the animation look more weighty