Beast

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.