Taxxon

Taxxons are giant sentient centipede-type aliens with an insatiable hunger. They will eat anything, even themselves, if given a chance. They volunteered to host Yeerks (sentient slugs that can take over the body of their host by appropriating their synapses etc etc, the Big Bad of Animorphs) in exchange for being fed lots of interesting things from multiple different planets.

Because of the hunger, all Taxxon recruits must undergo a permanent species change.