An Atmatar is an avatar drawn by forum member Atma. After being challenged on his status as "best new forumer of 2010" in the "fallen homies" thread, he took it upon himself to create an avatar for UsaSatsui, a crudely drawn rabbit saying "I'm UsaSatsui". He didn't use it, but the style was an instant hit, and Atmatars were soon all over the forums.
Atma was not the only one who did the Atmatars, but I can't remember who actually did each one.
Gallery of Atmatars
If you want to add your own, there's instructions hidden in the code. Just hit "Edit" and read them.