Shapeshifting Seducer

Logan: What do you want, an apology?

Mystique: You know what I want. (transforms from one woman to another) The question is, what do you want?

—X2: X-Men United

"I can assume any form I wish. Any form you wish."

—The Beast, Krull

When the Master of Illusion or character with Voluntary Shapeshifting wants to seduce someone, they have one very powerful tool in their arsenal: they can be anyone's "type". If they get their amorous advances rejected they will simply change their appearance until they hit paydirt. A little research goes a long way in this regard, and a lot of subtlety is needed to keep the target from figuring out the seducer is a shapeshifter. ... which is actually fairly rare for this trope, since the shapeshifter will basically be Showing Off the New Body in a superpower equivalent to a strip tease while the seducee is looking. If the fact that they're being seduced by a shapeshifter doesn't give them Shapeshifting Squick, they may actually find it really kinky and start making suggestions. Is likely to be involved in The Schlub Pub Seduction Deduction.

One of the quickest ways for this tactic to fail is for the shapeshifter not to do any research on the seducee or badly botch said research. This can happen if they take the form of a lost love, an ex, or their anti-type, the form likeliest to turn them off and make them run screaming.

Compare You Sexy Beast. See also Power Perversion Potential. Not to be confused with Shapeshifting Lover, where they only have one alternate form (and it's animal to human). Might be related to Shapeshifters Do It for a Change, however.

Examples of Shapeshifting Seducer include:

Anime and Manga

The Hentai Cosplay Sex Machine is based around this.
Tomoe, a Kitsune from Kamisama Kiss, does this for fun and profit.
Gantz has a particulary squicky example of this: Koki Inaba ended up having sex with a male alien disguised as Reika Shimohira.

Comic Books

Mary Shelley Lovecraft tried this in a Hack Slash story, morphing into a number of Cassie's past love interests while saying We Can Rule Together.
Runaways: Xavin is in love with Karolina, but he's not her type. Fortunately Xavin is a Skrull, with the ability to change his shape - including into a her.
There was a Spider-Man comic where The Chameleon tried to seduce Mary Jane by turning into different handsome men. It didn't go over well.
- Another comic had Electro visiting a shape-shifting prostitute, who offered to turn into Namorita, Sue Storm or the Scarlet Witch, among others. Spidey interrupted before we found out who Electro "fancied a crack at" this time...
Mystique of Marvel Comics has been known to use her shapeshifting for this purpose on occasion. The "Bizarre Love Triangle" arc involves her efforts to seduce Gambit by such a method - first by inventing the false identity of "Foxx" and, when that fails, taking Rogue's form instead - specifically in order to get him to cheat on Rogue and thus prove to Rogue that he isn't good enough for her.

Film

Epic Movie has a scene where the Mystique Expy seduces a character...and he asks her to get fatter.
Legend. While the protagonists are trapped in Darkness' jail, the fairy Oona tries to get Jack to kiss her by creating an illusion that she's Jack's girlfriend Lili.
Mystique in X-Men 2 and X-Men: First Class, which didn't really work for her either time until Magneto in First Class told her he preferred her without the shapeshifted appearance.
Excalibur. Merlin using his magic to change Uther Pendragon into the form of the Duke he's fighting in order to sleep with his wife.

Literature

The reluctant, eponymous succubus in Richelle Mead's Georgina Kincaid series relies on this power to seduce (mainly) men in order to get the life energy her kind need. Much is made of the fact that the seduction of morally upstanding individuals is made easier by simple groundwork, plus a lot of skill in 'seductive techniques'. Brand-new succubi apparently often lack this carefully-cultivated skill, instead relying on their physical attributes. When Georgina is assigned an apprentice to supervise, she is horrified that the newbie considers 6 ft blonde bombshells with breasts that defy physics to be every man's ideal. Said apprentice later reveals that she has much success with college boys, but can't understand why she can't snare moral men.
- Also, none of the several people Georgina - which isn't her actual name - has romantic entanglements with are attracted through any deliberate act of hers; they fall for her because of who she is. Whenever it is mentioned, none of them seem interested in the uses the shape-shifting ability might have in the bedroom. This is made more complicated by the fact that Georgina's 'default' form is not her original one either, and hasn't been for about 1500 years.
In A Night in the Lonesome October, the Thing In The Circle tries this on the protagonist-narrator in an attempt to get free of the Circle that imprisons it. It never quite succeeds in making him forget what it really is, and all it gets for its trouble is a series of deadpan snarker demurrals.
Iris in the first Xanth novel does this. She never hid her capacity for Magician-level illusions (although the more deceptive particulars are another matter), and, this being a Xanth novel, where every able-bodied male is horny as hell, it very nearly works.
What Saiman attempts with Kate Daniels, but never succeeds. His never-changing eyes make it Uncanny Valley for her.
Surprisingly averted in Harry Potter and the Half-Blood Prince. Ron points out that Tonks is nowhere as pretty as Fleur. Hello? Tonks is a shapeshifter, she can be the girl of anyone's dream.

Live-Action TV

At the start of Season 2 of Heroes, the illusionist Candace tried to seduce Sylar with, among other forms, hot twins, and himself. He kills her to try to steal the power for himself.
In The X-Files episode "Small Potatoes", Eddie Van Blundht used his shapeshifting abilities to seduce women, generally by posing as their husbands. In one case, he took advantage of a woman crazed about Star Wars by appearing to her as Luke Skywalker. This tripped him up. He knew about her obsession from years ago, and her social circle was pretty small.
The "illusion" variant of this is tried against Captain James Kirk in the Star Trek: The Original Series episode "Catspaw". Since he was already on his guard because of other strange occurrences, it doesn't work.

Oral Tradition, Folklore, Myths and Legends

In Classical Mythology, Zeus is depicted doing this several times when he wants to have an affair with a mortal woman, and often with little apparent regard for what a woman would find attractive. He carried off Europa in the form of a bull and met Danae disguised as a "shower of gold." (Which this troper thinks is odd, since they find nothing particularly attractive about a shower of gold, but after all there's no acccounting for taste.) He was in swan form when he chased and raped Leda (or Nemesis, Depending on the Writer). He used the trope's more usual form with Alcmene, though, by taking the shape of her husband so well she was totally fooled. The story that he (initially) hid his true form from Semele presumably includes him taking an attractive mortal human form with her, as well, although unlike Alcmene she wasn't married.
In one of the Norse myths, Loki talked the gods into letting a Giant rebuild the ruined Wall of Asgard—and promising him the Sun, the Moon, and Freyja if he finished before the deadline. The idea was to cheat him, except he looked ready to finish on time with the help of his super-strong stallion Svadilfari. The gods demanded that Loki get rid of that horse, so he transformed into a mare and ... distracted ... Svadilfari. Some months later, Loki gave birth to Sleipnir.

Tabletop Games

Changelings of Eberron are noted to be very popular in prostitution, except for those for whom the lack of fixed gender is unsettling.

Video Games

Ditto in Pokémon breed with almost everything.

Web Comics

In Oglaf a shapeshifter performs an all-too-obvious duck behind a tree for a not-very-clever change (very NSFW).
In Eerie Cuties 'verse, adult succubi have ability to control their shapes precisely enough to disguise as any individual humanoid and/or change their apparent age. The uses of which is rather obvious, given that their whole physiology is built around seduction. There are also reasons to suspect (but no clear confirmation) that the default shape of a succubus may be unconsciously affected by what others find attractive.