"Which beast in particular, Comrade Pillai didn't say. Searching for the Man who lives in him was perhaps what he really meant, because certainly no beast has essayed the boundless, infinitely inventive art of human haterd. No beast can match its range and power." p. 225 (CB)
Bruce Banner, a genius scientist with a potential beast inside his body. He was once doing a nuclear experiment in the desert, but a person accidentally walked into the experimental field. Bruce saved the guy from the nuclear shot, but he did not escaped from the Gamma radiation. Thus, the Hulk is formed. The Hulk comes out when his heart beat reaches 200, and being angry seems the easiest way to become the Hulk.