Egarus[/b] is a kind of nonsapient fungus. Originally it was native to the Abyss, but after nearly destroying a Prime+Material world it was banished to the Plane+of+Vacuum. There, it adapted to its new environment, learning how to feed on nothingness itself.
Planescape Monstrous Compendium Appendix III
Denizens of the Inner Planes (Planewalker.com download)