Beast is a transformation achieved by Gohan. It is unlocked by immense rage, and was achieved against Cell Max where the Half-Saiyan saw Piccolo faking his death. It is a direct successor to the Ultimate Gohan transformation, and as such, an extension of the Super Saiyan 2 transformation line, when in this form he acts savagely and attacks like a raging demon, but don't worry he's still a good guy protecting the ones he loves.