|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.
The monsters were the result of steroids administered by Coach Carl Marin. He managed to find the solution to old Soviet Experiments on the DNA of fish such as tarpon and mako shark to improve the performance of Olympic swimmers. Even though the formula was successful on the Sunnydale High Swim Team, extended use eventually led to the boys transforming into large, humanoid fish-like monsters. When transforming, the person's skin would completely shed off, leaving 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 monsters were released to live in the ocean and the remaining swim team members, including Xander Harris, underwent a number of treatments to reverse the process.