Apparently the hypnalis wriggled into a brief mention in medieval bestiaries. In our not-so-medieval bestiary (4 to be precise) it is the ether serpent, an intelligent, man-sized flying cobra that can shift between the Ethereal and Material Planes at will, and whose bite can force creatures back into the Material.
This last part is important, because hypnalises resent intrusions into their misty plane. Bestiary 4 details the various ways that ether snakes deal with interlopers, depending on each hypnalis’s age and particular temperament. Just why they behave this way is an interesting question, and may be tied in with whatever you imagine the hypnalis race’s origins to be…
The great serpentfolk kingdoms didn’t fall. They retreated into the Ethereal Plane. There they specially bred giant snakes and serus (see the Monster Codex) infused with magic to create a race that would both protect their secret and ward off the plane’s native inhabitants. (While phase spiders, xills, and the like can jump planes freely, they don’t enjoy the forced trip nor the Con damage any more than PCs do.) Adventurers who face a nest of hypnalises may be on the verge of discovering a new and terrifying nation of snake-priests.
Ether snakes are the Ethereal Plane’s immune system. When material creatures spend too long on the mist-shrouded plane, their presences begin to glow like beacons in hypnalis sensory pits. When adventurers need to make an extended stay on the Ethereal to find the victims of a xill abduction, they must first find a way to deal with the snakes. At least one wizard has managed to befriend the phasing reptiles enough to make his home on the Ethereal, and his tower is not far…
On the world of Maka, the Ethereal is less a plane than a fence, a band dividing the Material Realm and the Spirit World. The way between the two realms used to be porous, but three generations ago the People lost the ability to naturally step between the planes. The borders of the Ethereal began to stretch farther and farther apart…and the hypnalises arrived. The snakes seem eager to mock the People before they attack, and hint at being the vanguard of a far greater effort. Recent reports of ethereal mi-go seem to suggest that the snakes are telling the truth.
—Pathfinder Bestiary 4 154
Maka is apparently Sioux for “earth.”
I’m finally getting around to dropping my fussy “the” that I usually include with mentions of the numbered Bestiaries.