Satan appears! But it's not the Satan you recognise — this one has an hourglass figure and a cheeky smile, here to seduce our hero and steal his soul! (Pretty much never the other way around.)
This technically has a justification, or at least works as a blunt metaphor, since Satan is supposed to be tempting and sly when it comes to corrupting people, and being physically attractive would be a legitimate tactic in this case. The Bible even says that Satan will sometimes appear as "an angel of light".
Trickster gods in some mythologies are able to take whatever form they wish. Trickster gods are not always evil, but can be, and certainly aren't adverse to taking on a girly form just to mess with the heads of others.
Mythologically, Satan actually could take the form of a succubus to steal men's semen, after which he would turn into an incubus and pass it on to a woman, in order to have kids. How exactly this was supposed to transmit Satanic genes is undetermined.
May also be (particularly for female characters) the shoulder devil in a Good Angel, Bad Angel situation.
This is the ultimate form of Evil Is Sexy. See also Horny Devils. Contrast with Angelic Beauty. Not to be confused with Flaming Devil, although they can overlap.
Hild, while not Satan him/herself, is still the ruler of Hell C.E.O. of the Infernal Realm, and as she's Urd's mother, she also doubles as a Hot Shounen Mom.
Hagall: in the manga, when Hild is removed in a coup by her deputies, and the position is taken by Hagall, who is not a bad looker either.
The Devils of the Runaway Spirit Squad from The World God Only Knows are various degrees from cute (Elsea and Haqua) to hot (Nora).
My Balls: most of the demons are hot women, why would Satan be any different?
In Johnny the Homicidal Maniac, the devil has a few different scary, horned male forms, but assumes the form of a cute cheerleader girl because it's one of the things that Johnny likes the least.
On more than a few occasions, Loki in the Marvel Universe takes on a female aspect, often just to mess with Thor.
Lady Death from Chaos! Comics, Avatar Press, and Boundless Comics. After overthrowing Satan himself, she is, in fact, the ruler of Hell. Meanwhile, Purgatori from the same universe is actually a vampire, but, due to her Fallen Angel blood, she appears to look like a devil (wings, horns, red skin...no goat hooves, though).
Marvel Comics tends to portray Hela, ruler of the Norse land of the dead, as a beautiful, seductive woman. In mythology, she's often portrayed as half-alive half-corpse. In some continuities, she still is half corpse. Her costume is enchanted to give her the appearance of healthy beauty.
The recurring Crossroads Demon and Reaper may be in a different body every time, yet, somehow, they are always hot women.
Lilith often wears this type of meat suit. Or a terrifying, spoiledlittle girl. Whatever floats your boat.
The Twilight Zone had Satan turn up in several episodes, usually as a conniving businessman. But one episode, "Of Late I Think of Cliffordville", featured the Devil as played by Julie Newmar. She doesn't really try to seduce the protagonist, and, within that episode, she might simply be the Devil's usual form, but looking like Catwoman while bargaining for a man's soul has gotta count as an advantage.
Star Trek: The Next Generation: one episode had a presumed evil God-like entity claiming to be the devil in the form of a sexy woman. Good thing she didn't show up when Kirk was around.
Music
The video for Dimmu Borgir's song "The Sacrilegious Scorn" includes a painting of Baphomet morphing into a beautiful woman, though one with a deep male voice.
Although technically Baphomet is a woman anyway. At least, she is when the caduceus sitting on her lap isn't being interpreted as something else entirely, in which case Baphomet is a hermaphrodite.
Tabletop Games
In the Dragon Lance universe, one of Takhisis' various forms is that of "The Temptress". Basically, a really hot goth chick, although they don't describe it quite like that.
Touhou had, in Mystic Square back in the PC-98 days, the Goddess of the effective equivalent of Hell, Shinki. Not only a Designated Antagonist, since she had nothing to do with the calamity in the game, but also a (possible) Hot Shonen Mom of returning heroine Alice (canon isn't quite clear on that point).
Millenia from Grandia II. She very nearly manages to rape Ryudo...well, it would be rape, but it's hard to say.
In Princess Maker 2, it's possible for your daughter become the Princess of Darkness. She literally defeats Satan Lucifon and takes his place, and given her Hello, Nurse! status, this trope is played straight.
Strange Journey: Louisa Ferre is what happens when Lucifer is portrayed as a mysteriously seductive blond woman. Also, for the ladies, Louis Cypher of the other SMT games looks like a dead ringer for David Bowie, which, given his real life Memetic Sex God status, makes him a desirable being to either gender.
Webcomics
Narbonic, although justified in that the demon pulled the shape from Dave's fantasies. Further, it's later implied that the Devil herself is female.
Satan from Casey And Andy is a definite example - even though she loves him too, she's still Satan, and she doesn't play favorites...much. On at least a couple of occasions, she's seen subjecting the (frequently-dead) C&A to the torments of hell, answering Andy's complaints with "Waddaya want from me? I'm evil!"
In Jack, the devil appears to the reader in the form of the cartoonist, but to Jack, it seems he appears as a woman, since Jack refers to the devil as "she."
