Outcasts

Although not a true Centaur clan, the centaur outcasts are a group of exiled centaur. They are entirely made up of necromancers known as Outcast Necromancers.

Lands of Mystery claims that they are from the Magram clan.

Speculation
They may be lead by the Centaur Pariah, a centaur who wishes to unite the centaur clans. However, it is unlikely. It is more likely that he is just outcast too.