Power Instinct (豪血寺一族, Gōketsuji Ichizoku, "The Gōketsuji Clan") is a fighting game series created by Atlus started in 1993. The Power Instinct or Gouketsuji Ichizoku series (often abbreviated as PI or GI for short) is renowned for its more humorous take on fighting games, its music and its often-weird characters.
One of the innovations unique to the series was the introduction of the transformation feature. Several of the characters can change their appearance and fighting style by simply executing certain moves (usually a throwing move), though this has not been featured in every entry in the series. Other features include the ability to perform a double jump and attacking while dashing.
Source: Wikipedia, "Power Instinct", available under the CC-BY-SA License.