Dragon of Unhappiness
From Discworld & Terry Pratchett Wiki
Other civilizations in the multiverse have urban myths about creatures up to and including crocodiles which are bought as pets and flushed down the toilet when they become too big to handle. The urban myth also holds that what can be flushed down a toilet can find its way up again via the same route.
Dibbler has now added dragons to the list of creatures which have been treated in such a cavalier fashion, despite the angel of wrath represented by Lady Sybil Ramkin, who would no doubt be incensed at the thought... one Mr Passmore believed the sales patter too literally and it appears the Dragon manifested owing to his forgetting to keep the privy lid down when not in use. He should be up and walking properly again any day now...