David Anderson 
The Bullfrog Bully [EPUB ebook] 

Freddy was a big bully, until he was bullied himself! He was the biggest and meanest frog in the pond. But when he went to the ocean, he was just a tiny thing compared to the big fish and the shark. In the ocean, Freddy looked like a tasty snack to all the other animals. Maybe it wasn't such a good idea to be such a big bully after all, now that he knew how it felt to be bullied.