Angler fish
From Equestripedia, the Archives of Equestria!
Angler fish is a type of fish seemingly native to the southern portion of the Celestial Sea in Seaquestria. The only known angler fish so far is Theodore, a massive, building-sized fish able to substain an entire prison in his mouth. However, Theodore was cut from the My Little Pony: The Movie and thus isn't canon.
Sources
- The Art of My Little Pony The Movie, page 137