Actaeon
But after a minute the lady looked around and saw him! She was very angry, and immediately got out of the water and put on her dress. Unfortunately for Actaeon, this lady was the goddess Artemis, and Artemis hates for men to see her when she is in her bath. She was so angry that she pointed at Actaeon, and he began to turn into a deer! His feet and hands turned into little hooves, he grew horns, and he grew soft brown hair all over his body. He tried to tell her he was sorry, but it was no use.
Then the worst thing of all happened: Actaeon's own hunting dogs, thinking
he was a real deer, jumped on him and tore him to pieces.
(This story might seem a little unfair to you, but that's the point: gods do not operate by the same rules as people, and they can do things that are unfair if they want to. In fact, a look around at how many bad things happen to good people was enough to convince the Greeks that the gods did unfair things all the time. The story of Medusa is another along the same lines).
To find out more about Actaeon, check out these books from Amazon.com or from your library:

D'aulaire's Book of Greek Myths, by Edgar and Ingri D'Aulaire. (Look under Artemis).
Actaeon, the Unmannerly Intruder: The Myth and Its Meaning in Classical Literature, by John Heath (1993). By a specialist, for specialists.