Let's pee on the electric fence! It's time to talk Manic Pixie Dream Girls (MPDG). Rather than define this myself, check out Wikipedia's article.
The structure behind this phenomena is just fascinating.
The MPDG comes about in relatively few cases. You need a story about a main character in a rut, with another character who comes along with the energy to get him out of a rut (the manic pixie dream girl). Usually, this is a romantic comedy, but sometimes it's a life exploration film.
In order for this plot to work, you first meet and experience the doldrums of the protagonist. Since the protagonist is the point of view character, arguably the only character that is real, the plot revolves around him (or her). This means what whoever walks into the protagonist's life walks into his plot, into his gravitational well, so to speak. He's also sort of an everyman, so even though he's shallow, everyone who identified with him project their own character complexities onto him, so he seems more real to us because we sympathize. Finally, as we get to know the protagonist's trouble, we think that we get to know him, but we really don't.
Eventually a second character comes along to shake up his life. At first, this is unwelcome, because the character in a rut really is more secure in his safe run than in anything unsafe. This second character tends to be less real than the main character, so what's there has to really be very there. Because this character gets less screen time, the audience must meet and like this character immediately, identify this new this character as worthwhile, and root for a hookup to happen, all with inadequate information. If the intruding character doesn't meet that criteria, the audience wont' be happy. Because this all must happen so quickly, so much on the sly, the intruder's superficial qualities are more important than getting to know her character deeply.
If you were to give the second character as much screen time, you could lengthen the film (not always an option), take time away from the primary character (not always an option), and possibly risk the audience not liking the intruding character because they get to know her too well. There's nothing wrong with that idea for a more balanced structure, but the result is a different film, one that doesn't carry the same tempo as the structure above. In truth, the secondary character only really exists as a plot device for the primary character's growth and development.
Seen in this light, a manic pixie dream girl has nothing to do with sexism and everything to do with practical plot mechanics. Any sexism is merely an illusion, or an artifact of creating too many male led films.
At the end, the audience knows that our down and out lead character has gotten there when he accepts his manic pixie dream girl and lives happily ever after.
By this definition, the earliest manic pixie dream girl that I know of is Prince Charming from Snow White. We see the story through Snow White's eyes, her troubles, and her friends. When she's in trouble, Prince Charming shows up. He is instantly likable, gets almost no screen time, we root for him, and waltzes away with the girl, taking her out of her rut. By any objective analysis, Prince Charming has been reduced to a sexist stereotype, but really, he's just a plot device for Snow White's growth. He says, "You've gotten there!" The audience always needs something to say, "You've gotten there."