Hollow serpents are just that: empty snakeskins animated as undead. Which works thematically—they’re not just made from corpses, they’re made from shed empty skins, so they’re twice as empty as typical undead (even their original bodies rejected them!) and they hungrily seek to fill the void. GMs will note that CR 16, 20 Hit Dice undead don’t just pop out of mortal hands willy-nilly—these are monsters created by truly ancient rituals, artifacts, or the will of a god. Players won’t care about any of that; they’ll be too worried about the creatures’ lifesense and special abilities, all of which (channel, aura, coils, wilting, waves, strike, and 10 feats) punish PCs who get too close. But since hollow serpents are most likely to be encountered in the tight tunnels and temples of the serpentfolk (yuan-ti to you 3.5 fans), encounter distance is a luxury the GM can easily deny.
The upper levels of the Labyrinth of Shan-Voss have hosted minotaur paragons, dueling gynosphinxes, and at least one tataka rakshasa. But none venture into the maze’s third level, the Wilting Deep, where a hollow serpent circles endlessly, attended by serpentfolk huecuvas.
The Mouse Lord and the Cat Lord are two immortal spirits whose warring roles in children’s folk tales have imbued them with semi-divine status. Barely demigods, they still dwell in the mortal realm, however. An inveterate scavenger, the Mouse Lord guards his home with a hollow serpent he stole from the high temple of Bas-Seth itself. The Cat Lord sniffs that he could gnaw the head off the monstrosity in a heartbeat, but all his attempts to invade the Mouse Lord’s home have ceased.
Other reptiles also shed. Advanced hollow serpents are rumored to have been crafted from amphisbaenas (featuring extra bite attacks), behirs (lightning breath), couatls (flying), and even the fearsome imperial dragons.
—Pathfinder Bestiary 3 149