The Collaborative International Dictionary of English v.0.48:

Bestride \Be*stride"\, v. t. [imp. Bestrode, (Obs. or R.)
   Bestrid; p. p. Bestridden, Bestrid, Bestrode; p. pr.
   & vb. n. Bestriding.] [AS. bestr[imac]dan; pref. be- +
   str[imac]dan to stride.]
   1. To stand or sit with anything between the legs, or with
      the legs astride; to stand over
      [1913 Webster]

            That horse that thou so often hast bestrid. --Shak.
      [1913 Webster]

            Why, man, he doth bestride the narrow world
            Like a Colossus.                      --Shak.
      [1913 Webster]

   2. To step over; to stride over or across; as, to bestride a
      threshold.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bestride
    v 1: get up on the back of; "mount a horse" [syn: hop on,
         mount, mount up, get on, jump on, climb on,
         bestride] [ant: get off, hop out]

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "bestride":
   abut on, arch over, back, be based on, bear on, bestraddle, board,
   bridge, bypass, clear, climb on, command, cross, dictate, dominate,
   extend over, ford, get ahead of, get in, get on, go aboard,
   go across, go by, go on board, hang over, have the ascendancy,
   hop in, imbricate, jump in, jut, lap, lap over, lean on, lie on,
   lie over, look down upon, master, mount, outtop, overarch,
   overhang, overlap, overlie, overlook, override, overshadow,
   overstride, overtop, pass, pass by, pass over, perch, pile in,
   play first fiddle, predominate, preponderate, prevail, rely on,
   repose on, rest on, ride, rise above, rule the roost, shingle,
   shoot ahead of, sit on, span, stand on, step over, straddle,
   stride, surmount, take the lead, top, tower above, tower over,
   twist, wear the pants