and by very special I mean very very very very very very special. They award the privilege only to admins and I think mods (and maybe developers). I do not believe that knights have access to custom armatars.
@Firekiller55 we dont allow custom armatars, what you see are eighter:
(as Jacen said) knight/mod/admin/dev armatars.
(as miney said) rare armatars. These could be seasonal armatars (they are only available for a limited period of time, but as long as you dont change it you still use it) or legacy armatars, only available for a very few users who gain them in the past (no chance for new users cant get them). Also some mod users have armatars which was normal armatars, but now they changed to mod armatars (like the white rabbit), these are also no longer available (well, ok, they are... if you are a mod/admin...^^).
Some of the current available armatars are seasonal ones, as menitioned, only choosable for a limited period of time.
They follow after the "ninja-monkey", Raze 3 and summer themed... after summer we have also some from the Battle Pirates MMO, no idea if they are seasonal too^^
There is a way to have your own armatar here on AG, and thats the seasonal contests... if they fit to the theme of the contest of course.^^
The latest contest (raze 3 and summer, you can found the threads in our AMW section if you want to see how it works) are closed, but sooner or later (maybe halloween?) a new one starts.
If you want to participate in these contest (side-note: each entry is awarded with a merit, the winning submissions are also awarded with a non-game quest-badge!) check the AMW from time to time. They are also announced with a thread in the Tavern.
@Firekiller55 if im remember rgiht we have two ones. both was seasonal. a more colorful one (rare one i think^^) from spring 2013 and the greyish one from Halloween 2013.