The guecubu was originally a Chilean spirit—details on the Web are scarce, but apparently it was said to ride horses to exhaustion and otherwise cause torments in the mortal world. In gaming, the name (along with several other folk monsters like the dybbuk and the manitou) got applied to subraces of the loumara, a new race of demons introduced late in 3.5 (see below).
The Pathfinder version, from Bestiary 3, returns to the spirit realm. This guecubu is a vengeful spirit that has defied all the normal safeguards to ward against its return, making a body from the very earth itself.
After hearing him curse their names and swear vengeance, a party of adventurers did all they could to prevent the return of serial killer Bloody Michael—up to and including filling his mouth with holy wafers, cutting off his head, burning his body, and blessing and scattering the ashes. But some rages defy even fire and death, and soon they cannot stop at an inn or share a campsite without someone dying.
Sellswords comes across a wicked man hanging from a gibbet. So near to death, it does not matter whether or not they release him—they are the first targets of his guecubu when it rises.
Burying an evil man at a crossroads typically thwarts his spirit’s return—many clerics and oracles have a tale of a conversation held with a powerless shade they met in such a place. Burying an evil woman, especially a witch, druid, or lamia, is another matter. The sympathy between Mother Earth and her daughters is strong regardless of alignment or crime, and she will clothe and arm her daughters in the soil itself if their pain and rage are great enough.
—Pathfinder Bestiary 3 145
Several notes here:
1) I loved 3.5’s Fiendish Codex I: Hordes of the Abyss, and the loumara were a great concept. (Check it: Some gods ripped a layer out of the Abyss, and the plane itself reacted and destroyed them, then used their thoughts and dreams to spawn a new race of demons!!! Forgive me for the italics and multiple bangs, but that’s pretty hardcore.) That said, I’m against swiping the names of real-world mythic creatures and slapping them on completely fictional monsters. The loumara would have just as easily worked with made-up names, freeing the manitou and the dybbuk to get stats that fit their cultural origins. (More monsters plus more cultural diversity in gaming is always better. Typically Pathfinder gets a gold star for this—we have stats for the dybbuk now!—albeit with a hefty asterisk for the oread, sylph, and undine entries, which I’m more than a little out of sorts about.)
2) Gender and fantasy is a tricky thing. On the one hand, I want women in fantasy to be as tough (or as craven) as men, to get armor that covers their whole bodies, and to generally be equal participants in the world. That said, sometimes working with the female as “other” or with mythical feminine archetypes (the Earth Mother or the maiden/mother/crone trio, for instance) can lead to good scenario ideas—see above and the gorynych and green hag entries for examples. So far I think I’m keeping a decent balance, but feel free to weigh in if you think I’m doing a sloppy job in this arena, whatever your gender identification.
3) I’m a big believer in rewarding players for engaging in the GM’s world. That especially goes for folk remedies—if they bury one of their enemies at a crossroads so the corpse can’t rise up and find them, good for them! If they sprinkle a circle of salt around their beds to ward off inn wights (from Sword & Sorcery’s Creature Collection), give them a scene with a frustrated spirit circling their beds, unable to breach the barrier. Reward such engaged behavior in your players, again and again…
…And then give them the Harry Dresden treatment: After they’ve done everything right, and covered all the bases, send a guecubu after them to show them that no amount of hedge wizardry in the world can stop a real bad guy.