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.
Mastiff faced UK Champions Chaos 2, US robot Manta and German robot Ansgar in its only battle, which was also broadcast as part of German Robot Wars. It was immediately targeted by Chaos 2 and was flipped over several times, however no damage was caused internally. The pit was activated and Chaos 2 pushed Mastiff near the edge. However Chaos 2 tried to flip Mastiff into the pit, but only flipped it away and drove itself down instead. Mastiff didn't have time to recover as it was quickly pushed down the pit by Manta, eliminating them from the World Championship.