Thug was a human who lived in the Labyrinth.
Thug was guarding Fang's cell in the Labyrinth in late 1996 when he was knocked out by Dr. Sevarius. Shortly thereafter, he was abducted from the Labyrinth by Sevarius and Fang and was injected with a mutagenic formula, turning him into a human-crocodile mutate.  After being freed by the Redemption Squad, Thug returned to live in the Labyrinth and has continued to be a trusted compatriot of Talon. 
Before his mutation, Thug was a muscular African-East Indian male with a shaved head. After being mutated he greatly resembles a bipedal crocodile. It is unknown what his real name is.
Behind The Scenes
The inclusion of a crocodile mutate was done in part for reasons related to the story (Sevarius creating armored mutates), but also to continue the Gargoyles tradition of adapting legends and myths. In this instance, the urban legend of alligators living in the sewers of New York City is alluded to.