Zorphic crash landing on Earth.
The Morphic race is an alien species that can shapeshift at will, which is introduced in Issue 35 of the Invader Zim comic series. The prime example of this is Zorphic the Morphic (better known as Madness), who came to Earth to peacefully observe things. And by "observe", he meant "smell".
Throughout the series, Zim has encountered the shapesfiting alien more than once, believing him to be a real chihuahua, and in some cases an actual threat to his "mission". As the Irken seems to fear him, as seen in "Walk of Doom" and "Walk for your Lives" to name a few episodes, this was probably why, as Issue 34 reveals, Zim had him imprisoned on Moo-Ping 10. In Issue 35, Zorphic reveals his true nature to Zim so that they can team up to escape the prison.
Trivia
- It's unknown what the Morphic race really looks like since Zorphic is only seen disguised as a chihuahua for most of his time on Earth in both the series and the comics. The flashback to Zorphic first taking that form features a squid-like creature, however it's unknown if that's a Morphic's true form or simply something Zorphic transitioned through in the process of becoming a dog.