|Although this article is based on canonical information, the actual name of this subject is conjectural.|
Gill Monsters were a race of half-human/half-fish creatures. They were the result of the steroids administered by Coach Marin who managed to find the solution to old Soviet Experiments on the DNA of fish such as tarpon, mako and shark to improve the preformance of Olympic swimmers. Even though formula was successful on the Sunnydale High Swim Team, extended use eventaully led to the boys transforming into large, humanoid fish-like monsters. When transforming the person's skin would completely shed off and all that would be left was a mindless beast with a taste for human flesh and the capability of breathing both underwater and on land.
After the death of Coach Marin, all the Gill Monsters were released to live in the ocean and the remaining Swim Team members, including Xander Harris, that hadn't turned yet had to undergo a number of treatments to reverse the process.