What? That’s totally a unicorn.
Unicorns don’t exactly need any introduction. Even the most casual fantasy fan knows their evolution from confused references in ancient Greek bestiaries to medieval virgin detectors to My Little Pony sorceresses.
Pathfinder has walked back the unicorns’ problematic traditional obsession with virginity, which I think is a good thing—now you can have a Rat Queens-style character without having to pass up your shot at a unicorn steed. It’s a good thing, that is, unless you really want them to be virgin-obsessed. Slut-shaming is bad in general, but strictures—and the breaking of them—lead to adventures. (See the first adventure see below. )
My only other thought to add is that you have to drop unicorns in early. Like a lot of other magical steeds, they're going to need pretty heavily enchanted barding or class levels to stay viable for upper-level parties. But for the low-level party facing TPK, the sudden appearance of a blessing of unicorns could be the difference between life and death. If you prefer to play in good-aligned campaigns or reward hard role-playing choices, it’s also a way to give your good PCs a little benny to make up for all those orphans they’re always having to save…
If you need a unicorn enemy, you can always make them intolerant defenders of Nature, or switch up their alignment/spell-like ability combos to craft neutral and evil breeds. Several articles in Dragon Magazine explored that option, as I recall (issue #190 especially). And in the Scarred Lands setting, the savage Hornsaw unicorns (think Clydesdale crossed with a sawfish; see the Creature Collection) were actually that setting’s default unicorn, so successful were they at displacing the core species.
Poachers hunting a unicorn catch Maeve, the baron’s daughter, planning to ransom her after her virginity lures a unicorn to them. Maeve would dearly love a unicorn to show up, but her virginity is more mythical than the beast itself, having quite gleefully vanished in the arms of a jongleur at the Tennight Masque when she was but 14. If adventurers accept the baron’s mission to retrieve his daughter, they will find she is more trouble than the poachers, as she refuses to return home until she has at least glimpsed a unicorn—because if her father finds out she is not a virgin, he will disinherit her in favor of one of her eight sisters in a trice. Whether or not she lets the party in on her plight or just sabotages their every effort to leave the forest depends on her estimation of their sympathetic natures.
Evelyn Pendergast sought a unicorn’s bond but was rejected, shaming her family. She has since become a ranger and hunts the beasts with alacrity. She is a specialist at drawing them out of hiding. Tormenting useful fey like brownies or domovoi (see Pathfinder Adventure Path #67: The Snows of Summer) is her preferred tactic to force a unicorn into a confrontation.
As magical beasts, unicorns respond to—and reflect—their habitats. Neutral or evil unicorns are often the result of extreme trauma or magical pollution. The unicorns of Dagger Dell weep blood constantly from their manes. Their touch can cure blindness but also inflict light wounds. The unicorns of the Hissing Forest have black hides and are constantly surrounded by swarms of insects; sometimes they ally with gryphs and other vermin-friendly abominations.
—Pathfinder Bestiary 269
Back when I was in high school, one of the earliest issues of Realms of Fantasy (maybe the very first one) had a story about a suburban family’s struggles living next to an enchanted wood. One of the running gags was that every time the daughter went outside, unicorns tried to lay their heads in her lap, much to her embarrassment. But by the end of the story, it is strongly suggested that she has…taken care of that problem.
Edit: Sorry for the late entry. Original post from a very bad day: Came home to 4 inches of water in my basement courtesy of a burst pipe. Unicorn post will happen another day—thanks for bearing with me!