The Trotterdragon is mentioned in the Prologue of How to Betray a Dragon's Hero. It is a small dragon that must resemble Toothless (in the Books). One is abused and used as a fake item in the King's Things which is necessary to pass onto the Island of Tomorrow. That particular Trotterdragon was released and went to live with the Guardians on the Isle. The perpetrator is punished with "Death by Airy Oblivion".

"UG the Uglithug has removed the teeth from a poor little Trotterdragon in order to pretend it is the real toothless dragon from prophecy."
How to Betray a Dragon's Hero