The Denebian Slime Devil is a non-sapient species of amphibious predatory creatures native to the planet Deneb IV.
Biology
Denebian Slime Devils inhabit the seas and wetlands of Deneb IV and can vary in size from a few centimeters to 1.2 meters and weight up to 26 kilograms. Their average lifespan is of 17 years. They are red in color and have four slender limbs. The hindfeet appear to be webbed, probably to aid in swimming. These creatures are named for their extreme aggressiveness and are armed with long claws and sharp teeth. The long tendrils on their head are used for echolocation and they also have a large single nostril on their faces, used for smelling. The skin covering the Slime Devil's head, besides being very tough, also contains reflective minerals which can reflect a phaser fire. They hunt fish-like organisms, mostly, but will attack much larger creatures if given the chance, especially to protect their eggs.
Behind the scenes
Denebian Slime Devils were only mentioned once in Star Trek, when Korax, a Klingon, insulted Captain Kirk by calling him a Denebian Slime Devil. The closest to actually seeing them in canon happened in the Deep Space Nine episode "A Man Alone", when an image of the creature, along with several other species, is briefly seen on a computer screen. All the information about this species, as well as the illustration, come from non-canon reference books Star Trek: The Worlds of the Federation and The Starfleet Survival Guide.