P-Chan is the transformed verson of
Ryoga Hibiki from the
anime series
Ranma 1/2.
When Ryoga is splashed with cold water, he turns into a little black pig. This happens because he fell into the Spring of Drowned Pig at the
Jusenkyo training grounds in
China (also called
the Cursed Springs). And when Ryoga touches hot water, he turns back into his human self.
Ryoga got the name P-Chan when Ranma's fiance
Akane Tendo found Ryoga in his transformed state. She instantly fell in love with the little piglet, and adopted him as a pet, calling him P-Chan ("P" for pig and "chan" meaning little). Akane has no idea that P-Chan is actually Ryoga.