Trying to locate a children's book (Title/Author unknown)?

The Aminal is about a boy who finds a green creature and then proceeds to describe the creature to a friend and then that girl tells another person, who tells another person, and so on. In each telling the creature is described as bigger and badder than the previous. This is probably not the book you were looking for, but was the only one I could come up with that was about a boy finding a green creature.