The Anura Unit was formed by Toad Man and his prototypical brother, Frog Man. Hoping to vie for their own stint of world domination (presumably after a flaw in their programming code caused by some sort of physical damage incurred upon them somehow), they would eventually come across Dr. Kojiro's Poison Man, and after what may have been reprogramming to make Poison Man more "free-willed" himself, the three would go on to hire freelance French scientist named Dr. Grenouille to construct another five robot masters to fill out their ranks.