Gary Gaunt was originally just a criminologist who had been developing a formula that was supposed to turn rabbits into vicious fighting machines. It was his hope that these experiments would allow him to find a way to counteract things like rage and aggression in people.
Unfortunately, some mobsters attempted to steal Gary's work and tried to kill him with an arranged car accident. Gary managed to survive the accident, but his disorientation from the ordeal caused him to accidentally drink his own experimental formula.
The formula caused Gary to transform into a savage, monstrous version of himself at night, during which he would try to direct his anger at criminals. During the day, Gary continued his crime-fighting, while also searching for a cure with his girlfriend, Jean Carson.
In the Marvel Adventures continuity, Gary Gaunt had a similar origin, except it was revealed that during World War II he had been used as a living weapon by the United States against Germany. He was dropped into German territory at night, and he would rampage until day, at which point he would revert back to his human form. Gary would then be forced to hide until night and the process would repeat itself. Gary Gaunt endured this hit-and-run tactic for two years.
After the war, Gary was finally able to develop a formula that could keep him from transforming at night. Using this, he settled down and eventually retired.
Several years later however, some small-time thieves robbed the retirement home that Gary had been staying at, and ended up taking his formula. Realizing that he wouldn't have time to make a new batch of the formula before nightfall, Gary was terrified of what he might do if he transformed again after so many years. Luckily, his old friend Sun Girl was able to find Nova, Invisible Woman, and Black Widow of the Avengers and asked them to help Gary retrieve his formula before it was too late. Nova stayed behind in order to prevent Gary from harming anyone if he were to transform and go berserk, while Invisible Woman and Black Widow tracked down the thieves and the formula. It ended up being a close call, as Gary slowly began transforming into his alter-ego, but luckily Nova was able to get him his formula in time before anyone got hurt.
Powers and Abilities
In his human form, Gary Gaunt is an accomplished criminologist with extensive scientific knowledge.
When he transforms into his savage state, Gary loses most of his intellect and reasoning, but he also becomes considerably larger, stronger, and more durable. According to Sun Girl, Gary is maybe a little more than half as strong as the Hulk when he transforms. This transformation is unfortunately involuntary and is triggered by the night. Gary likewise automatically reverts back to his human self during the daytime.
Even though the transformation is involuntary, Gary is capable of preventing it by drinking a special formula made of very rare ingredients. This formula must however be administered on a regular basis.