Foul Mouth

A massive hostile beast that favors the dark and cold of the unknown. Few have seen them and lived to describe them in detail, but they're always described as a lengthy lizards with sharp claws and teeth. With a mouth that opens well past what should be their jaw and instead reaches their stomachs, they could easily swallow most things whole. They breath out a poisonous gas that kills small creatures. Larger beings, however, can survive breathing in the gas, but it often leaves them confused and makes it difficult to see.

Habitat
They prefer remote caverns of frozen mountains and will go as deep as they can into them. Those that have gone looking for these beasts are certain that they will even dig deeper into the mountain to be further from the surface.

Biology
Their poisonous gas is thick and difficult to see through, most who breath it in will be nauseated and experience minor hallucinations. Their exact size is difficult to determine, the smallest is reported to have been the size of a horse while others claim they’ve seen one that rivals the size of dragons.

Sociability
They prefer living alone but will allow lesser monsters to occupy the areas that are closer to the cavern entrance. Their mating habits are a mystery even to the most dedicated researchers... The ones still living, anyway. These creatures are overall a very uncommon sight. If a human manages to get past the multiple efforts of warding them away, a foul mouth will attack them mercilessly and do everything it can to leave no survivors.

Trivia
Lesser monsters and sprites seem to know better than to anger a Foul Mouth, but it only gets truly hostile towards humans.