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A white-haired ant monster. He can create tornadoes and manipulate soil and use it to bury people. He attacked a playground and its surrounding areas until Kotaro and Joe arrived to stop him. Kotaro transformed into Kamen Rider Black RX to fight him (with Shadow Moon secretly observing the battle). When Ant Runt created a boulder, Kamen Rider Black RX became Robo Rider. However, Robo Rider proved useless against Ant Runt as the monster clogged all of Robo Rider's joints with dirt. After transforming into Bio Rider however, he easily maneuvered against Ant Runt causing the monster to retreat and saving everyone who had been buried. He and Gedorian later crashed the battle between Bio Rider and Shadow Moon. Ant Runt trapped Bio Rider and Shadow Moon in an anthill. Ridron freed them and Bio Rider turned into water to attack Ant Runt. He then reverted to Black RX and destroyed Ant Runt with the Revolcane. Antrunt was later revived by Maribaron as part of the Spirit World Kaijin Army.