The Butt Brain

Yinrih have a caudal ganglion, which is a secondary brain located at the base of the tail whose original purpose was to control the tail, but which has evolved secondary functions as a sort of backup for the main brain. Yinrih can withstand head injuries that would render a human dead with no long term consequences as the main brain can rebuild itself from information stored in the caudal ganglion--a very useful feature if you fall on your noggin from a tree on a regular basis.

There's a neurological disorder caused by a partial severing of the connection between the two brains where the tail acts on its own accord.