Invader ZIM Wiki
Brain leech

Zim with the Brain Eating Parasite.

The Brain-Eating Parasite was a giant parasite that Zim either created or exported from his home planet in the episode "Backseat Drivers From Beyond the Stars" to destroy the human race. The parasite, once released onto the surface, would eat the brains of all humans.

Zim asked GIR to make sure the parasite didn't escape, but GIR did not obey his master and allowed the controls that kept the parasite asleep to go haywire. The parasite escaped into the base, and tried to eat Zim's brain. Zim somehow survived the event and it's unknown what happened to the parasite afterwards.