Decepticon
Decepticons are one of the two major factions of Transformers, a race of robotic sentient alien races that hail from the planet of Cybertron which exists...somewhere in the infinite multiverse, probably. Often considered "evil forces", the Decepticons have been portrayed as anything from good guys to well-intentioned extremist to genuinely awful beings to things we can't really talk about on a child-friendly wiki.
Funnily enough, Decepticons have have more of an impact on My Little Pony media than the Autobots.
Fiction
Ask Sideways
Apparently, the Decepticon Sideways once got into a fight with Discord and Maccadam's Old Oil House and somehow won.
Transformers vs. G.I. Joe
Megatron was somehow transported to the world of Unicorn when he instead wanted to find Unicron, the eater of worlds and his charred body was licked by Twilight Sparkle.