They were formerly high school students who were members of the Sunnydale High Swim Team, but were turned in to fish-like creatures after constantly exposing themselves to steroids created by their coach, Carl Marin, that were meant to improve their swimming skills. Carl made the drugs by finding the solution to old Soviet Experiments on the DNA of fish such as tarpon and mako shark. While it worked, the side effects resulted in them becoming mindless creatures with a taste for human flesh resembling the Gill-man. He subjected the teens to this formula by mixing it in with the steam used in the sauna. The transformation would happen if they were exposed long enough.
Before the transformation occurred, signs of them changing would be sudden changes in behavior, such as aggression, as well as a foul odor surrounding them. As they transformed, their skin would completely shed off, leaving only the mindless beast within, with the ability to breath both underwater and on land.
After their transformation, many believed that the boys were killed by some mysterious monster who skinned it's victims. After their mutations, their coach kept them contained in a part of the sewer system under the utility room. Carl threw Nurse Greenliegh, who had helped him with the experiments, in to the sewer with them to be eaten because she had questioned his methods. After Buffy Summers discovered Carl's plans he threw her in to the sewer to satisfy his boys' "other needs". As they try to gang rape her, Xander manages to intervene and help Buffy escape, while Carl is accidentally thrown in, only to be eaten by his own boys.
After their coach's death, the monsters managed to escape Animal Control to live freely in the ocean.
Known Gill Monsters
According to Xander Harris, he was once considered a freak with "Jimaca Breath" who waxed his back. A month later, after winning some meets, he became popular and the best swimmer in the team. During a beach party to celebrate the team's victory, he was bullying Jonathan Levison by dunking his head in a bucket of ice water because he wanted to join the team, but Buffy intervened. He walked off with his friend, Gage Petronzi to talk about how weird Buffy was, but Dodd stopped, seemingly hypnotized by the ocean waves, while Gage kept walking. After he smelled a noticeably foul and malodorous odor, he turned around, only to find Dodd missing. After he left to go looking for him, Dodd immediately transformed into a Gill Monster and walked away into the sewers.
He was portrayed by Jake Patellis.
Cameron briefly dated Buffy and seemed kind enough until he tried to rape her in his car, resulting in her giving him a broken nose and getting into trouble with Principal Snyder for "leading him on". He then hit the sauna before heading to the cafeteria to turn into a Gill Monster.
He was portrayed by Jeremy Garrett.
Gage was a student in Jenny Calender's computer class which, after her death, was then taught by Willow Rosenberg who noticed Gage's tendency to misbehave such as playing solitaire in on his computer during class, not doing homework and having unacceptable test scores by not showing up to class. Willow marked him accordingly against the wishes of Principal Snyder, who wanted her to unfairly pass him to a D grade standard so he could remain on the swim team and win State Championship.
After Buffy suspected the team mates were being murdered, she began following Gage as the assumed next victim. Annoyed by this, Gage confronted her at The Bronze before getting attacked by Angelus after having a brief conversation with him about Buffy. Luckily for him, the steroids given to him by his coach made his blood distasteful, causing Angelus to stop feeding and spit it out before Buffy rescued him. After he escaped, a grateful Gage asked Buffy to walk him home because he was scared. The next day, after waving to Buffy during swim practice, he was attacked by a Gill Monster in the locker room, before turning into one himself (much to his shock and surprise).
He was portrayed by Wentworth Miller.
Sean was present in the sauna when the undercover Xander Harris was looking for the steroids that he believed transformed the students. When he asked the teammates were they were, Sean informed him that the steroids were being absorbed from the steam in the sauna. He later became a Gill Monster due to the dosage he took, briefly confusing Cordelia Chase into thinking Xander was the monster before realizing he was just in another room. Sean attacked both of them, but they managed to escape.
He was portrayed by Shane West.