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 a cunning seducer, 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 female form just to mess with the heads of others. If their form is tailored to suit the precise tastes of whoever they're trying to pull, they're a Shapeshifting Seducer.
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.
Maou-sama is the leader of the demons, ultimate enemy of mankind and a gorgeous woman. Subverted in that she is closer to Prometheus than to Lucifer and wants a sustainable peace between their realms.
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.
X-Men: Azazel toyed with other people making them believe he was a devil, and he also seduced Mystique.
In the 1990's version of Vampirella's comic, her sexy mother Lilith rules Hell. (Luckily for Vampi when she dies and ends up there; her mother gives her a mission to fight vampires on Earth.)
In Death Isn't All It's Cracked Up to Be Satan, while female and reasonably attractive, was more mischievous than evil. Similarly, Hell was more about punishment and redemption than torture and temptation.
The 2000 remake of Bedazzled (2000), with Elizabeth Hurley as Satan. In the original version, Peter Cook wasn't too bad looking either.
The Ninth Gate featured the devil, aka Irene Adler, described by the main character as the most beautiful woman he had ever seen.
There was a movie called Hunk in 1987 that was sort of a parody of the Deal with the Devil plot. While the Devil was portrayed as a balding, overweight, middle-aged man, his servant - who actually presented the contract to the protagonist - was female and a knockout.
The recurring Crossroads Demons may be in a different body every time, yet, somehow, they are always hot women. Invoked, since they're specifically using those hosts to make their deals more palatable to their male clients, which is sealed with a kiss.
We get to see some male crossroads demons in later seasons, including their king, Crowley.
Satan himself is played by Mark Pellegrino, and the muscular Jared Padalecki.
In "The Song Remains the Same", Dean has a dream in which he's serviced by a stripper in a red devil's costume.
Dean: I take it all back. I love the Devil.
(Devil stripper is joined by a blonde in an angel's costume)
Dean: Now that's what I call peace on Earth.
Just to add to it, Anna, an actual angel who Dean had previously slept with, then shows up. She's visibly irked by his dream.
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.
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) mother of returning heroine Alice (canon isn't quite clear on that point).
In Princess Maker 2, it's possible for your daughter become the Princess of Darkness. She literally defeats Lucifon and takes his place, and given her Hello, Nurse! status, this trope is played straight.
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."