Source : Webster's Revised Unabridged Dictionary (1913)

Mounter \Mount"er\, n.
   1. One who mounts.

   2. An animal mounted; a monture. [Obs.]

Source : WordNet®

mounter
     n 1: a skilled worker who mounts pictures or jewels etc.
     2: someone who ascends on foot; "a solitary mounter of the
        staircase" [syn: {climber}]