Yeerk

A yeerk is a fictional extraterrestrial species from the book and television series Animorphs. It is a parasite that takes over the mind of another being by way of infestation through the ear canal. The process of infestation by a Yeerk is painless: a numbing chemical is secreted to deaden the ear canal itself so that the sluglike Yeerk will not cause any physical harm to the host.

After the process of infestation, the Yeerk flattens itself into what has been described as "hammered-out Silly Putty" and sinks into all of the cracks and crevices of the brain. From there, all control is submitted involuntarily to the Yeerk. The host has no control over itself anymore.

And, since the Yeerk has access to all parts of the brain, it is able to pass flawlessly as its host (now called a Controller) so that nobody else suspects that there is, in fact, a secret invasion of their planet.

Known hosts: Gedd, Hork-Bajir, Taxxon, human, Andalite, Leeran, Garatron, horse