The Collaborative International Dictionary of English v.0.48:

exerciser \ex"er*ci`ser\, n.
   1. One who exercises.
      [1913 Webster]

   2. one who supervises the exercise of animals, such as horses
      or dogs.
      [PJC]

   3. a device used for exercising muscles; as, a thigh
      exerciser.
      [PJC]

WordNet (r) 3.0 (2006):

exerciser
    n 1: sports equipment used in gymnastic exercises [syn:
         gymnastic apparatus, exerciser]