The Lice Queen is a massive, monstrous lice, which Countess von Verminstrasser believes is the source of all lice on Earth. She eventually tracks it down after it causes an outbreak at the Skool, and destroys it with a firearm that weaponizes Zim's skin, which is apparently toxic to lice. It makes its only appearance in "Lice".
The Lice Queen looks very bloated. However, this is just because of the egg sac containing her offspring. She is actually able to detach from it, and move swiftly.