The sarcastic fringehead (Neoclinus blanchardi) is a very bellicose saltwater fish that

The sarcastic fringehead (Neoclinus blanchardi) is a very bellicose saltwater fish that can open its mouth widely and use it as an assault weapon when engaging in territorial battles. They joust by pushing their distended mouths against each other, as if they were kissing, to determine dominance.
.
(TITLE FROM REDDIT r/Damnthatsinteresting)

Oof, I posted a picture of this, but now that I see a gif it looks even more terrifying.