The Chihuahua, otherwise known as "Madness", is a small dog with a chipped ear, one brown iris and a stitch across its head. It has made several appearances over the course of the series. It has been dubbed "Madness" by the fans, seeing as Zim screamed "MADNESS!" upon seeing the dog in its first appearance in "Walk of Doom". The chihuahua might be a reference to a chihuahua mentioned in Johnny the Homicidal Maniac, when Nny screamed to a survey man that "This little chihuahua started following me".
In the audio commentary of "Walk of Doom", Jhonen said it was planned as a running gag that actually didn't work out.
When Zim turned a corner, the Chihuahua was there. He immediately screamed "MADNESS!" and ran in the other direction. He also was inexplicably seen on top of a building right before Zim and GIR fell into a dumpster.
When Zim realized that the "Saucer Morons" have taken the Voot Carrier, on one of the pedestals as an "alien artifact" was the chihuahua.
When Zim and GIR arrived at a neighbor's door, GIR was tossing eggs everywhere, cooking them on the sidewalk. The Chihuahua then appeared and started eating the sunny-side-up eggs.
The Chihuahua stole meat from a questionable meat vendor, and was dragging it across the street in front of the City Cesspool when he met Zim in his grandma disguise. Zim panicked, yelling "Dog with meat! Dog with meat!!", and got GIR's leash caught on a nearby lamppost, which toppled him and sent the slow-motion explosion flying away into the city.
When GIR shook off dirt in his house form, Courtney Lilly could be seen walking the Chihuahua.
When Dib realized that he was missing his briefcase, the Chihuahua found it in the alleyway he left it and dragged it away with his mouth.