Fenrir's Clique: The Chain Gang
The members of this clique spend a lot of time—well, okay, all their time—in detention. And compared to some of his fellow chain gang-members, the brand of imprisonment Fenrir has to deal with doesn't seem all that bad. (At least he's still got all his internal organs.) Fenrir's in permanent lockdown because a wise-woman has prophesied that he'll kill Odin during Ragnarök, and Odin's not taking any chances. But other members of the chain gang have actually already behaved badly, and now, looking at an eternity of detention, are probably wishing they hadn't.
The Minotaur, a.k.a. Asterion, was actually born to punish his stepdad, Minos. Since Minos wouldn't sacrifice the bull Poseidon gave him, Poseidon decided that he'd make Minos's wife have sex with a bull instead (which sounds more punishing for the wife than for Minos). The Minotaur, a man with the head of a bull, was the result. Since he ate people and generally wreaked havoc in Crete, Minos imprisoned him in a giant labyrinth and now feeds him an appetizing diet of human sacrifices.
Apep is all bad. Period. He is the god of everything evil in ancient Egypt, including chaos and darkness. Some Egyptians believe that the only reason the sun rises every day is because Apep is trapped somewhere below the horizon. Sometimes Apep manages to break free even during the day, causing thunderstorms and solar eclipses. To make sure Apep didn't take over for good, Egyptian priests wrote a how-to book called The Books of Overthrowing Apep. It includes instructions for all kinds of different punishments to direct at him, including spitting, fettering, and our favorite, "Defiling Apep with the Left Foot."
Prometheus managed to make Zeus really angry a couple of times, but when he returned the fire to humans that Zeus had taken from them as punishment, it was the last straw. For Prometheus, Zeus devised what is probably the worst punishment, in any mythology, ever: he tied him to a rock and allowed an eagle to eat his liver all day. Since he was immortal, Prometheus's liver regenerated every night. So this gross torture went on forever, or at least until Heracles untied him. But that's another story.