The frill shark has a large mouth with 25 rows of 300 sharp teeth, six pairs of fringed gill slits, and a long, eel-like body that can grow up to 6 feet long. It's referred to as a "living fossil" because it's a member of one of the oldest living shark species.
The giant isopod, known scientifically as Bathynomus giganteus, is the largest known member of the isopod family. It is very closely related to the small pillbugs that you can find in the garden. It is a carnivorous crustacean that spends its time scavenging the deep ocean floor. Food is extremely scarce at these great depths, so the isopod has adapted to eat what ever happens to fall to the ocean floor from above. It will also feed on some of the small invertebrates that live at these…
Scully: On the old mariner maps, the cartographers would designate unexplored territories by simply writing "Here Be Monsters". Mulder: I've seen the same thing on maps of New York City. — The X-Files, "Quagmire"
One of the scariest creatures ever to live in the ocean, this Devonian fish could grow up to 33 feet long, had an armored face, and likely had one of the strongest bites in history! It used a beak-like mouth instead of teeth to devourer its prey. It was one of the largest of the Placoderms, a group of armored fish that are now extinct.