This brain is one of the brains that can be found in the Sierra Army Depot and be used to build a mobile body structure for Skynet. However, using the abnormal brain makes Skynet so stupid that it can only act as a pack horse. Although it can be issued commands, it must be forcefully disbanded by destroying it.
On the fourth floor of the depot, retrieved from the storage system.
Behind the scenes
The abnormal brain and its acquisition are a reference to the movie Young Frankenstein, where Igor accidentally destroys the optimal brain for the eponymous scientist's monster and takes a brain labeled "Abnormal" (which he reads as "Abby Normal") instead.