Muk
With the round head and broad mouth, I image muk to be something amphibian like, may it be a tail or tail-less one. This kind of animals is also heavily associated with poison.

This creature releases a substance from its body that is poisonous and smells very strongly. When this secrete comes in contact with air, it swells (like hagfish and their slime together with saltwater). Not only doest this make it uneatable but also protects its skin from drying out. Still, this creature need to drink a lot instead and can often be found around water.
The sludge is water based, and to keep it away from entering its eyes, the muk releases an oily substance from glands near the eyes.

The muk can almost eat anything organic. Rotting material from the bottom of lakes seems to be a favourite, but the broad mouth hints that it can hunt however, and it’s believed that they sometimes swallow other creatures that are slower then itself whole; it isn’t like something smelling that strongly and that is so slow can sneak upon prey very easily.
Its slow speed is not only due to the slime and its weight but also the fact that its diet contains very little energy.

Muks often causes issues in sewers as their thick poision clog up pipes, but what attracts them there it's the waste and the water.

Made for :3