Behemoth is one of the 369 spawns of the Ogdru-Jahad that was introduced for the film series, making it the film exclusive Ogdru Hem. It had a gourd-shaped, worm-like body with numerous tentacles of unspecified number on each points, with lower tentacles far larger than the ones on the head. Behemoth also had numerous pair of black eyes across its underbelly.
However, the tentacled "head" on the upper point encases an actual retractable head with two pairs of eyes and long tentacle-like tongue.
Ever since Grigori Rasputin pledged his servitude to Ogdru Hem, a named Ogdru Jahad, who turned him into his mortal agent, he had the Behemoth residing within his body to keep an eye on him as well as acting as the mad mystic's "God".
The Behemoth's presence within Rasputin's body resulted in his inability to die, as not only the mystic would eventually be revived, but each resurrection would lead the Behemoth to become stronger.
The Behemoth ultimately stepped in to deal with Hellboy and Agent John Meyers and the unconscious Elizabeth Sherman in the climax of the film when Rasputin reveals to have it within his body the entire time. It emerges from the stab wound in Rasputin's body and grows to gigantic size, whereupon it crushes Rasputin and Ilsa Von Haupstein as punishment for their failure in manipulating Hellboy to free its parents, the Ogdru Jahad. Hellboy has a tough time grappling Behemoth at first, but manages to destroy it by forcing himself to get eaten and detonate a belt of grenades, causing it to explode from the inside.
The Behemoth was likely inspired by Sadu-Hem from the comics.
When Rasputin explains his plans to Trevor Bruttenholm, the Behemoth's presence within Rasputin can be noticed when it moves its tentacles under Rasputin's skin within his hand, foreshadowing its arrival in the climax. The Behemoth can be noticed writhing around again inside Rasputin's head, moments before emerging from his body.