The Mastermind Rat resides in Isaac's unused lab under his house in the events of DFoS. He is an altered test subject who was terribly deformed and has an enourmous IQ level and is extremely powerful. Abe must defeat him in order to get a piece of the teleporter to get to the Realm of Induction.
In DFoS 2, the Mastermind Rat reappears as the main antagonist and utilizes Sinisto's power for ultimate evil without his knowing. He has arranged an army of various mutated animals and takes over Prematon. Ganondorf is the only survivor and escapes and teams up with Abe to help defeat him. The Mastermind Rat plans to use his power to absorb the entire world and everything on it to become the ultimate being.