The Lesser Blackbacked Seadragon is a dragon species from How to Train Your Dragon (series) mentioned in How to Speak Dragonese when Gobber the Belch gives a class to the vikings in training about how to raid a peaceable fishing boat in the middle of a fog.

"Peaceable fishing boats are always followed by flocks of Lesser Blackbacked Seadragons, hoping for a bite. All you have to do is follow the racket they make and you'll have found yourself a boat."
How to Speak Dragonese

No description is given, though by the above statement, they may function like seagulls. They have "Seadragon" in their name, so it might be inferred that they are classed as Sea Dragons.