Sai Mutant known as in English is Rhinoceros Mutant: A rhinoceros monster. It can charge, ram into things, has super-strength, and it attacks with its horns.
Kamen Rider Black
In a plan to have humans fight each other, Gorgom used a grasshopper serum in a champaign. To help those people who already had the serum and to prevent Kamen Rider BLACK from interfering, the Rhinoceros Mutant was unleashed to help them and it first brutally killed four gang members. In the first battle, it damaged Battle Hopper (whom was revealed to have quick-healing powers). In its second battle at Honmoku Docks, he broke off the Rhinoceros Mutant's front horn and Battle Hopper rammed the Rhinoceros Mutant. The Rhinoceros Mutant was wounded by both before being destroyed by the Rider Kick.
Kamen Rider World
Rhinoceros Mutant was revived along with four other monsters, Goushima, Agito, Garai, and Zu, by Shadow Moon. These five revived monsters were defeated by Kamen Riders ZO and J.