The Collaborative International Dictionary of English v.0.48:

Protean \Pro"te*an\, a.
   1. Of or pertaining to Proteus; characteristic of Proteus. "
      Protean transformations." --Cudworth.
      [1913 Webster]

   2. Exceedingly variable; readily assuming different shapes or
      forms; as, an am[oe]ba is a protean animalcule.
      [1913 Webster]
      [1913 Webster]

WordNet (r) 3.0 (2006):

protean
    adj 1: taking on different forms; "eyes...of that baffling
           protean grey which is never twice the same"

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "protean":
   able to adapt, adaptable, adjustable, allotropic, alterable,
   alterative, changeable, changeful, checkered, diversiform,
   ever-changing, flexible, fluid, heteromorphic, heteromorphous,
   impermanent, kaleidoscopic, labile, malleable, manifold,
   many-sided, metamorphic, metamorphotic, mobile, modifiable,
   movable, multifarious, multifold, multiform, multiphase, multiple,
   multiplex, mutable, nonuniform, permutable, plastic, polymorphic,
   polymorphous, proteiform, resilient, rubbery, supple, transient,
   transitory, unsettled, unstable, unsteady, variable