Beast by Ally Kennen
In the depths of a reservoir lives a monstrous creature, unknown to anyone except the teenage boy who feeds it. Six years ago it was a baby. Now it's grown, and its rusting cage can't hold it much longer... the Beast is Stephen's biggest secret. His life in foster care is getting worse, and he's in trouble with the police. All the odds are against him, but his efforts to free himself of the Beast make him a hero that readers will never forget.