Satan is a catfish.

Satan is a Texas catfish.

Satan eurystomus is better known as the "widemouth blindcat", a type of eyeless albino catfish that lives deep in the ground (hypogean or stygobitic), in the water filling the rock interstices of the deep Edwards Aquifer under San Antonio, Texas.  Feeding on any creature it encounters in its underground travels, the widemouth blindcat is a top predator that can grow up to 13.7 cm (about 5.5 inches) long.  Lacking eyes, it must detect its prey by sensing electrical disturbances with its lateral line and barbels.

We know about this creature due to its being pumped, along with irrigation water, from five Artesian wells around San Antonio.  In 1938, Carl L. Hubbs, an ichthyologist at La Jolla University near San Diego, California, was invited to San Antonio's Witte Memorial Museum to view several blind catfish which had been pumped out of nearby wells.  Carl Hubbs is a giant among ichthyologists, a prolific author on fishy subjects.   When he determined that one of the fish was an unknown  species, he gave it this whimsical name because its deep, dark abode (or a somewhat drier version) was once thought to be the location of Hell. The fish's somewhat repulsive appearance probably helped matters.

Genus Satan
Family Ictaluridae
Order Siluriformes
Class Actinopterygii
Superorder Pisces
Subphylum Vertebrata
Phylum Chordata
Kingdom Animalia
Domain Eucarya

The Hubbs Story

Aldemaro Romero's Lab,

The above website gives a citation for the original paper naming the infernal fish:
Hubbs, C. L. & R. M. Bailey. 1947. Blind catfishes from artesian waters of Texas.
Occasional Papers of the Museum of Zoology, University of Michigan (499):1-15.

The Handbook of Texas Online