Phirasaurus is a piranha-dinosaur monster. His main ability was releasing a poisonous breath. The second Phirasaurus had an unbelievably strong body, able to shrug off rounds of a cannon, and special techniques called Ultra Punch and Ultra Kick, similar to the Rider Punch and Rider Kick. The first Phirasaurus had the human guise of a bus driver, and was killed after using his death gas ability to kill the people in a bus. Phirasaurus was a project from Shocker commander Maya, and she quickly looked for a stronger person to be altered. The second Phirasaurus was pro-wrestler Noboru Kusaka. He was captured, brainwashed and had his body altered, becoming the perfect Phirasaurus. The new Phirasaurus was able to match and defeat Kamen Rider #2 with his Ultra Kick. Brainwashed, Kusaka returned to the wrestling world as Satan Mask, and attacked his own brother when he tried to speak with him. He eventually faced Rider #2 in a wrestling ring. During the battle, he accidentally killed his commander Maya with his poisonous gas. In the end of episode 17, Kamen Rider #2 and Phirasaurus exchanged a Rider Kick and a Ultra Punch. Afterwards, both fell on the ring, and Kusaka returned to human form, recovering his memories.