The Collaborative International Dictionary of English v.0.48:

Dynast \Dy"nast\, n. [L. dynastes, Gr. ?, fr. ? to be able or
   strong: cf. F. dynaste. See Dynamic.]
   1. A ruler; a governor; a prince.
      [1913 Webster]

   2. A dynasty; a government. [Obs.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

dynast
    n 1: a hereditary ruler

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "dynast":
   anointed king, chief, chieftain, crowned head, emperor, grand duke,
   high chief, imperator, king, king-emperor, kinglet, lord paramount,
   majesty, monarch, overlord, paramount, petty king, potentate,
   prince, prince consort, royal, royal personage, royalty, ruler,
   sovereign, suzerain, tetrarch