Heh.

http://www.youtube.com/watch?v=VoBjp8wOQ4E

Squeeee!

Hey, are you the one who originally designed her as a RATO?
Also, what is/was Chem-chan? Can't find nothing online

Wow, this was a long time ago... After some brief digging, it seems Chem-chan was created by/for /tg/ as a character in the Warhammer 40K setting. (https://1d6chan.miraheze.org/wiki/Chem-chan).
While I mostly used to hang around the Drawthreads on /co/, I did pop over to /tg/ a few times, so this must have been one of those times. I'm fastidious about dating these pics and this one is from 14 October 2009. I honestly don't know if there were any rat!Chem-chans before that, so... maybe I was the first? ¯\(°_o)/¯

Feels like it, I was never able to get who commissioned the character from other artists but as it seems, it don't belong to a single person, unless it does which at that point would assume it be you (the rat version that is)