Children's book.

Im around 30 years old, I think I read it around 10 or 11
It was about a ugly brown dog who wanted to change. The dog found a witch living in a wood in a tree house, she offered to help him out and changed parts of him into a elephant, moose, zebra and other animals. He ran away frightened and found a circus and got treated as the freak show. Later on it pans out that some farmer helps him out and he turns into a white stallion and attracts loads of attention.

If any knows the title or at least the publishers that would be cool thanks!