The Collaborative International Dictionary of English v.0.48:

Mutable \Mu"ta*ble\, a. [L. mutabilis, fr. mutare to change. See
   Move.]
   1. Capable of alteration; subject to change; changeable in
      form, qualities, or nature.
      [1913 Webster]

            Things of the most accidental and mutable nature.
                                                  --South.
      [1913 Webster]

   2. Changeable; inconstant; unsettled; unstable; fickle. "Most
      mutable wishes." --Byron.
      [1913 Webster]

   Syn: Changeable; alterable; unstable; unsteady; unsettled;
        wavering; inconstant; variable; fickle.
        [1913 Webster]

WordNet (r) 3.0 (2006):

mutable
    adj 1: capable of or tending to change in form or quality or
           nature; "a mutable substance"; "the mutable ways of
           fortune"; "mutable weather patterns"; "a mutable foreign
           policy" [syn: mutable, changeable] [ant:
           changeless, immutable]

Moby Thesaurus II by Grady Ward, 1.0:

133 Moby Thesaurus words for "mutable":
   able to adapt, adaptable, adjustable, alterable, alterative,
   ambitendent, ambivalent, at loose ends, brittle, capricious,
   changeable, changeful, changing, checkered, choppy, corruptible,
   deciduous, deviating, deviative, deviatory, different, disorderly,
   divaricate, divergent, diversified, diversiform, double-minded,
   dubious, dying, ephemeral, erose, erratic, evanescent,
   ever-changing, fading, fence-sitting, fence-straddling, fickle,
   fleeting, flexible, flitting, fluctuating, fluid, fly-by-night,
   flying, fragile, frail, fugacious, fugitive, impermanent,
   impetuous, impulsive, inconsistent, inconstant, indecisive,
   infirm of purpose, insubstantial, irregular, irresolute,
   irresolved, jagged, jerky, kaleidoscopic, lubricious, malleable,
   many-sided, mercurial, metamorphic, mobile, modifiable, momentary,
   mortal, motley, movable, mugwumpian, mugwumpish, nonconformist,
   nondurable, nonpermanent, nonstandard, nonuniform, of two minds,
   passing, perishable, permutable, plastic, pluralistic, protean,
   proteiform, ragged, resilient, rough, rubbery, shifty,
   shilly-shally, short-lived, slippery, spasmodic, sporadic,
   subject to death, supple, swaying, swinging, temperamental,
   temporal, temporary, ticklish, transient, transitive, transitory,
   uncertain, undecided, undetermined, undurable, unenduring,
   unequable, unequal, uneven, unorthodox, unresolved, unsettled,
   unstable, unsteady, unsystematic, ununiform, vacillating, variable,
   variegated, variform, various, varying, volatile, wavering