The Collaborative International Dictionary of English v.0.48:

Dismount \Dis*mount"\, v. i. [imp. & p. p. Dismounted; p. pr.
   & vb. n. Dismounting.] [Pref. dis- + mount: cf. OF.
   desmonter, F. d['e]monter.]
   1. To come down; to descend. [Poetic]
      [1913 Webster]

            But now the bright sun ginneth to dismount.
                                                  --Spenser.
      [1913 Webster]

   2. To alight from a horse; to descend or get off, as a rider
      from his beast; as, the troops dismounted.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dismount \Dis*mount"\, v. t.
   1. To throw or bring down from an elevation, place of honor
      and authority, or the like.
      [1913 Webster]

            Dismounted from his authority.        --Barrow.
      [1913 Webster]

   2. To throw or remove from a horse; to unhorse; as, the
      soldier dismounted his adversary.
      [1913 Webster]

   3. (Mech.) To take down, or apart, as a machine.
      [1913 Webster]

   4. To throw or remove from the carriage, or from that on
      which a thing is mounted; to break the carriage or wheels
      of, and render useless; to deprive of equipments or
      mountings; -- said esp. of artillery.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dismount
    n 1: the act of dismounting (a horse or bike etc.)
    v 1: alight from (a horse) [syn: unhorse, dismount, light,
         get off, get down]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "dismount":
   alight, buck off, climb down, demolish, depose, deracinate, detach,
   disassemble, disconnect, disengage, disjoin, dislodge, dismantle,
   dismember, disunite, get down, get off, land, light, perch,
   root out, root up, separate, settle, take apart, take down,
   tear down, throw off, touch down, unhorse, unplace, unsaddle,
   unseat, uproot