Upon discovering a target, the Proto-Tyrant quickly approaches and attacks with its abnormally developed claws. The claw on its right arm developed into a shape reminiscent of an equestrian's spear while its left arm developed into a smaller form with several large claws. Its body suffered severe decomposition as a result of the virus corroding the body, with its skin marred with lesions, missing skin and necrotic patches. Like the final model, it also possessed an exposed heart on its chest, and even its spine became exposed as its skin decayed and fell off, making it vulnerable to attacks from the front and back.
Despite the imperfections in its appearance, its physical functions were equal to the completed model. It possessed astounding vitality and aggressiveness which were reminiscent of the completed model's form following removal of its limiter. Despite its combat abilities being inferior to the completed model, it was still a deadly organism. However, its lack of intelligence rendered it uncontrollable. Despite its inferiority in several aspects, it could be said that its high level of ferocity and frank brutality made it somewhat more deserving of the name "Tyrant" than its successor.
This Tyrant was defeated twice by Rebecca Chambers and Billy Coen over the course of Resident Evil Zero, and later destroyed by Albert Wesker during the Resident Evil: The Umbrella Chronicles scenario "Beginnings".