Phirasaurus is a piranha-dinosaur monster.
His main ability was releasing a poisonous breath. The phirasaurus had an able to shrug off special techniques called Ultra Punch and Ultra Kick, similar to the Rider Punch and Rider Kick.
The first Phirasaurus was killed after using his death gas ability to kill the people in a bus. Phirasaurus was a project from Shocker commander Maya, and the second Phirasaurus was pro-wrestler Noboru Kusaka. He had his body altered, becoming the perfect Phirasaurus.
The new Phirasaurus was able to match and defeat Kamen Rider #2 with his Ultra Kick. Kusaka 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. Kamen Rider #2 and Phirasaurus exchanged a Rider Kick and a Ultra Punch and Kusaka returned to human form, recovering his memories.