Selkiemore

"If you thought a pig that spent three days in a pile of shit smelled bad, try testing your nostrils with warm Selkiemore guts…"

A selkiemore, called by folk Selkie Maw, is a water-dwelling creature that feeds on plankton.

Selkiemores in culture
While hardly anyone has seen a selkiemore, folk tales describe it as giant beasts with thousands of teeth that line a mouth capable of swallowing entire villages.

While in fact selkiemores are just plankton feeders, fear of them may lead the people to hire a witcher to deal with them. One such beast was slain by famous Geralt of Rivia, who managed to cut his way out of the creature's interior after being swallowed.

Trivia

 * Selkies are shapeshifting seals from Scottish mythology.
 * In real world, whales are an example of plankton feeders that were previously portrayed in legends and myths as toothed monsters capable of swallowing people alive.