A flea-like monster. He can launch a stinger from his mouth and shoot lasers from its eyes. Mundayganday was the one responsible for killing Kyoko Matoba's parents. He, Gedorian, and the Chaps were attacking a reservoir in an attempt to stop the flow of water into the city because Maribaron was delivering water to people. When Kotaro, Reiko, and Kyoko arrived at the reservoir and were attacked by Mundayganday Kotaro transformed into Kamen Rider Black RX and fought him until Gedorian and the Chaps trapped him a slime-like substance shot from their guns. Kamen Rider Black RX became Bio Rider and destroyed Mundayganday's exo-skeleton with his Bio Blade. He then reverted to Kamen Rider Black RX and destroyed Mundayganday with his Revolcane.