The Oct Imagin is an Imagin and an antagonist in episodes 41 and 42 of Kamen Rider Den-O.
Kamen Rider Den-O
He made a contract with Shōko Hazuki, who wanted Yuto Sakurai to be brought to her. Though Shoko wanted Yuto to be brought to her alive, much to the Oct Imagin's frustration, after the Oct Imagin travels back in time, he becomes able kill Yuto without any penalty and takes control of the ZeroGasher to use against Zeronos. He was later destroyed by the combined efforts of Liner Form and Zero Form.
However, a piece of his tentacle infected the AuraLiner from Liner Form's finisher and built itself into a giant hydra-like monster formed by numerous time tracks. After a lengthy battle, the combined firepower of all the time trains and the KingLiner ultimately destroys it.
Several Oct Imagin later appear as part of Kai's Imagin army in the final arc.
OOO, Den-O, All Riders: Let's Go Kamen Riders
In the alternate timeline where Shocker achieved world domination, the Oct Imagin was one of several monsters who served the organization in the year 2011.