Invader Skoodge is the short, fat invader usually shown with stains on his clothes. Whatever his physical appearance, though, Skoodge is tough - he conquered the Slaughtering Rat People of Blortch and he survived being fired out of a cannon from the Massive into the planet surface. He later survived an attack from the meat-thirsty Hogulus on Hobo13.
I have read in the unproduced scripts that Skoodge was going to come to Earth and live with Zim which I'm not too sure would have worked out well, but generally I liked Skoodge.
Did you say slaughtering rat people?