Yeerk
The Yeerks are a fictional alien species from the Animorphs series of young adult books by K. A. Applegate. They are parasitic slug-like creatures that seek to conquer other sentient species by infesting their brains and controlling their bodies. A Yeerk enters its host through the ear canal, typically while the host is asleep, and attaches itself to the brain stem. Once attached, the Yeerk assumes complete control, forcing the host to act according to the Yeerk's will. The host's consciousness is not destroyed but rather suppressed, trapped within its own mind, aware of everything but unable to act independently.
Yeerks come in various castes, with the Andalites and other species often serving as primary hosts. Their