Sinnertwin is a minor antagonist in Generation One of the Transformers franchise. He is a member of the Terrorcons and transforms into an orthrus. He frequently takes pleasure in causing destruction and frequently looks for things to destroy. Ironically, he's terrified of small creatures.
Sinnertwin first appeared in the third season episode "Grimlock's New Brain", along with the other Terrorcons. They were among the Decepticons sent by Galvatron to salvage anti-electrons in Unicron's head for an attack against the Autobots. They were defeated by the Technobots, newly created by the temporarily hyper-intelligent Grimlock.
Sinnertwin and the Terrorcons attempted to steal a recreator machine for the Quintessons in "Money is Everything". In "Call of the Primitives", he was among the animal-form Transformers summoned by Primacron and drained of energy by Tornedron, but was revived when Grimlock defeated Tornedron.
In "The Return of Optimus Prime", Sinnertwin (along with the rest of the Terrorcons) was part of a raid on Mark Morgan and Gregory Swofford's laboratory to steal a new metal. He lasted appeared in "The Rebirth" as part of Galvatron's final attack on Cybertron.
Sinnertwin first appeared in the Headmasters miniseries as one of the Decepticons serving Scorponok on Nebulos. He was involved in the first battle with the Autobot Targetmasters, which ended with the Autobots retreating on realising the battle was destroying the nursery they were trying to protect.
Sinnertwin appeared in the main Marvel US comic during the Underbase Saga. In Issue #49, he was among the Decepticons who charged to assist Scorponok when he ended up in a conflict with Ratbat's Earth Decepticons. He was later part of the Autobot-Decepticon alliance against the Underbase-powered Starscream. He and the other Terrorcons joined a unit led by Fortress Maximus and Soundwave who defended Buenos Aires but were deactivated during the battle.
The post-movie version of the character appeared in "Time Wars", where he and the other Terrorcons assisted Soundwave in an attempt to drain Earth's energy. After being stopped by Kup, Arcee and Red Alert, they pursued the Autobots back to 1989, displacing Horri-Bull and several other present day Decepticons into limbo. Caught up in a battle with a time-displaced Galvatron, they quickly retreated back to their own time.