I don't have the module with me, but I believe the Beast of Averoigne is a shapechanger that can take on a pseudoserpentine form.
I found this here:
http://www.classichorrorstories.com/texts/beast.txt
"Passing among the ancient trees that towered thickly behind Perigon, he thought that he discerned a light from the windows, and was much cheered thereby. But, going on, he saw that the light was near at hand, beneath a lowering bough. It moved as with the flitting of a fen-fire, and was of changeable color, being pale as a corposant, or ruddy as new-spilled blood, or green as the poisonous distillation that surrounds the moon."
"Then, with terror ineffable, Gerome beheld the thing to which the light clung
like a hellish nimbus, moving as it moved, and revealing dimly the black
abomination of head and limbs that were not those of any creature wrought by God. The horror stood erect, rising to more than the height of a tall man; and it swayed like a great serpent, and its members undulated, bending like heated wax. The flat black head was thrust forward on a snakish neck. The eyes, small and lidless, glowing like coals from a wizard's brazier, were set low and near together in a noseless face above the serrate gleaming of such teeth as might belong to a giant bat."
"This much, and no more, Gerome saw, ere the thing went past him with its nimbus flaring from venomous green to a wrathful red. Of its actual shape, and the number of its limbs, he could form no just notion. Running and slithering rapidly, it disappeared among the antique oaks, and he saw the hellish light no more."