oligarch

from WordNet (r) 3.0 (2006)

oligarch
    n 1: one of the rulers in an oligarchy

from The Collaborative International Dictionary of English v.0.48

Oligarch \Ol`i*garch\, n.
   A member of an oligarchy; one of the rulers in an
   oligarchical government.
   [1913 Webster]

from Bouvier's Law Dictionary, Revised 6th Ed (1856)

OLIGARCHY. This name is given to designate the power which a few citizens of 
a state have usurped, which ought by the constitution to reside in the 
people. Among the Romans the government degenerated several times into an 
oligarchy; for example, under the decemvirs, when they became the only 
magistrates in the commonwealth.