Sarcastic fringeheads are small but ferocious. They live alone in burrows and they’re EXTREMELY aggressive, opening their mouths to scare off any creature that approaches their territory. This includes those of their own kind; sarcastic fringeheads are commonly seen wrestling each other with their mouths pressed together, usually until the fish with the smaller mouth gives up.
Yet they look somewhat harmless and unassuming with their mouths closed:
…I’ll stick to petting sea hares.