Scrolling a little way down this persons feed I saw that the Chihuahua’s name is Morty and he suffered an accident or has a condition and his owners are rehabilitating him. The bouncing is his way of moving but you can see he is hunched and spine is rigid. However the fact that he has been able to find a movement pattern that works means he has gained mobility and is probably a happy little chappy because he can now get about.