At first, cueros appear to be just another fantasy batoid species. But with their relatively high intelligence (Int 6, well within human range) and the ability to speak Aklo, it’s clear that something more is going on here. Such scores put them in the company of cloakers and trappers (the mature form of female lurking rays). In fact, cueros may even be degenerate cloakers, having lost some of their power when not in the Shadow-energy-bathed subterranean realms. (Actually, in the Golarion setting the opposite is likely true: cueros were probably the original stock from which the aboleths created cloakers.)
In any case, cueros are riverbank hunters, ambush predators, and potential moon worshippers. They may not have magic or the malice of their cloaker kin, but they make river travel in the tropics—already plagued by disease, piranhas, alligators, and drakes—still that much more dangerous.
Cueros have a taste for ungulates…and centaurs. After the death of a foal, a centaur tribe is desperate for revenge, but with the monsoon season in full swing, there is too much danger from quicksand, flooding, and the cueros themselves for the horsefolk to organize a counterstrike. Adventurers who aid the tribe will find their next few quests free of wandering monsters, as they are discreetly shadowed by centaur rangers who see that they remain unmolested.
An augury to seek “the prayerful ones who swim like bats and call to the moon” leads eventually to a tropical lagoon filled with cueros. Diving into the right tide pool on the solstice or the equinox leads adventurers to a silver city on the moon populated by cloakers and a half-mad and truly tarnished silver dragon.
Since the death of Dyana Foss at the hands of charau-ka, scholars and field researchers once again begin employing adventurers on a regular basis to protect expeditions. National Geographick hires a party of adventurers to accompany a top loremaster into the Amazon to study the mysterious one-eyed, one-armed, and one-legged fachens. There is the usual run-in with skum at the port—Brazil has long been aboleth country—but the adventurers don’t really get a taste of what a dangerous task they've undertaken until they are attacked by a pack of cueros not even one day out of Manaus.
—Pathfinder Bestiary 5 62
Last week I got zero useful hits when I searched for this monster, but this week I’m getting directed to all kinds of South American cryptid sites. Google is weird.
By the way, “cuero” apparently means leather, skin, or animal hide in Spanish. It also apparently means prostitute in the Dominican Republic. I haven’t explored that particular nuance of the word…but cueros do have Int 6 (again, well within human range)…so maybe some aberrations gonna be aberrant? If you want your next jungle adventure to reach truly Apocalypse Now levels of dark…well, there ya go.
Also, is there such a thing is post-ipation? I’ve had this thing half-written since Wednesday—Wednesday!—and it would not come out.