Xacarbas are super-cobra-like creatures of riddles, reversals, and confusion—no wonder, given that on Golarion they serve the demon lord of magic and forbidden lore. In fact, their name is a twist on his own, echoing the palindromes and formulae that were the basis of the earliest crude magical writings.
Even the xacarba’s flawed shapechanging is a riddle—in their humanoid forms they maintain one serpentine feature to clue in the observant. (And if the observer acts on that knowledge, so much the better—the xacarba will have backup plans within backup plans, and meanwhile the perspicacious meddler will have identified herself as a target.)
A disguised xacarba uses its change shape and spell-like abilities to send three factions of spellcasters against each other, manipulating the mages’ guild, a consortium of sages of serpentfolk lore (and serpentine bloodlines), and an extended family of enchanters against each other. As the spellcasters maneuver against each other, the xacarba is free to loot their libraries and temples, stealing scrolls and weakening the city’s magical defenses.
A loremaster finds evidence of a forgotten magical symbol lost to history. She thinks it can be successfully reconstructed by studying the tripartite glyphs on the back of a living xacarba.
Less-popular boys in an elite boarding school have been experimenting with occult techniques: Ouija boards, automatic writing, hypnosis, and the like. Over time, one phrase keeps cropping up, a twist on “Abracadabra” that goes “Abba Xacarba…Abba Xacarba.” Eventually the boys’ incantations summon the Snakefather, a xacarba of great power. He quickly charms or tortures all of them into his service. And in this latter era of automobiles, telephones, and typewriters, there are few adventurers with the magic to stand up to an outsider of any magnitude, let alone the (15 HD or more) Abba Xacarba…
—Pathfinder #18 88–89 & Pathfinder Bestiary 2 288
Welcome to X, ladies and gentlemen. Better buckle up, because we’re not going to be here long.
The original Pathfinder entry for the xacarba listed it as a demon, while the Bestiary 2 stat block doesn't. That raises the kind of “What is a demon—a denizen of the Abyss or something more?” question that taxonomy-minded GMs love.
Also, despite all those books and links I mentioned yesterday, I forgot to mention that Mythic Adventures has a mythic wyvern, for those of you who like your dragons with power lifting and a swallow whole attack.