Filler art
From The Heretic Knowledge Vault
Drawings by Poe (or sometimes others) that are inserted into the thrice-weekly sequence of strips when real-life circumstances don't allow him to complete a strip to his satisfaction in time for publication. (Think of them as Fan service, or if you prefer, omake.) Many feature characters from the strip; others feature Poe himself and/or Impy in various predicaments; others just come from Somewhere Out There.
An example of filler art involving catgirls appears here. It isn't clear whether filler art featuring actual ES characters should be considered canon for purposes of character development, etc. In this particular case, the art itself makes it clear that it's not canon, which is really kind of a pity, at least for Jon and Sarine.
A thread-like bit of recurring filler, called "Fun with Familiars" and starring Ellis and Rape-kun, is getting going. Look here for a sample.
Not to be confused with Wallpaper, which isn't part of the Monday-Wednesday-Friday sequence and has another goal, namely encouraging people to donate.
