Krindlekrax by Philip Ridley
Ruskin Splinter dreams of being a great actor but the school bully, Elvis Cave, gets the part instead. Ruskin seeks solace with the kindly school caretaker who tells him the story of long ago when Ruskin's father 'borrowed' a crocodile from the zoo where he worked and then lost it. The crocodile - Krindlekrax - lives deep in the sewers where the school caretaker used to work and, against all the odds, and, against all the kindly townspeople who try to tell him he can't do it, Ruskin saves the town from Krindlekrax and becomes the hero after all.