From Dragon Quest Wiki

Great beasts of the Mesozoic era, Dinosaurs are often classified as dragons in the dq games, though some exceptions exist for the more bird-like examples. Because the games got their start just before the Dinosaur Renaissance of the late 80's, both scaled and feathered species are present. Additionally, this category includes other ancient life forms not technically classified as dinosaurs by real life paleontologists for conveniences' sake.