Gustav

Gustav is the name of a crocodile that helps Braulio, Legantir and Blucher train young knights in Justin and the Knights of Valour.

Powers and Abilities
Gustav may not posses any magic abilities, but one of Braulio's inventions allows him to fly and "breath" fire; so the young knights that the monks train would know what it is like to fight a dragon.

The device was used to fight Heraclio's men, when Blucher went to challenge a former knight.

Hiccup Horrendous Haddock III
Hiccup would help the crocodile know what it is like to be a "dragon" and tinker with the device he wears; to make it more compile for him or to make him more dragon like.