Mastiff was a steep grey basic wedge shaped robot armed with a flipping arm and two side spikes to prevent it from being stranded on its side when flipped. On its wedge, it featured the image of an angry dog's face, like that of Bulldog Breed, while the robot also had the capability of running on its back if it was flipped in that position. Despite being listed as having no self-righting mechanism, the flipping arm was capable of successfully self-righting Mastiff at least once, although Mastiff had a high ground clearance that was easily exploitable.
It was immediately targeted by Chaos 2, being flipped over before self-righting. Chaos 2 proceeded to flip Mastiff a few more times, before causing the Italian machine to become immobilised. The pit release was activated and Chaos 2 pushed Mastiff near the edge, flipping it onto its back once again. However, Chaos 2 tried to flip Mastiff into the pit, but only flipped it away and drove itself down instead. Mastiff was then swiftly pitted by Manta, eliminating them from the World Championship.