Kython

Kythons (not to be confused with Kytons, or chain devils) are distinct from the other Fiends in that they did not originate on any of the lower+planes. When a group of fiends (the Galchutt, from Monte Cook's Chaositech and Ptolus) were trapped on the Prime Material Plane, they tried creating more of their own kind through magical means. The results were eyeless reptilian creatures with insectoid traits and neutral evil traits. As the kythons matured, they took on varied forms. None of them were loyal to the fiends that created them. Because kythons originated on the Material Plane instead of the Abyss (or another lower plane), they are also called earth-bound demons. Kythons are only interested in eating and breeding. They have spread rapidly across the Material Plane. The current hierarchy of kythons, from the weakest to the strongest is: broodlings, juveniles, adults, impalers, slaymasters, and slaughterkings. Eventually, with more time, kythons will grow into newer and more powerful forms.

Behind the DM Screen
Kythons were originally created for Monte Cook's Ptolus campaign, based on some gaming miniatures he had bought, and were added by him to the Book of Vile Darkness absent the context of the Galchutt, who wouldn't appear until later on in Chaositech.

Monte Cook originally planned on perhaps renaming them so their name was not quite so similar to kytons, or chain devils. See http://montecook.com/arch_ptolus1.html, as well as other episodes of Monte Cook's Ptolus campaign to see how they were originally used.