The Collaborative International Dictionary of English v.0.48:

Mocker \Mock"er\, n.
   1. One who, or that which, mocks; a scorner; a scoffer; a
      derider.
      [1913 Webster]

   2. A deceiver; an impostor.
      [1913 Webster]

   3. (Zool.) A mocking bird.
      [1913 Webster] mockernut

WordNet (r) 3.0 (2006):

mocker
    n 1: someone who jeers or mocks or treats something with
         contempt or calls out in derision [syn: scoffer,
         flouter, mocker, jeerer]
    2: long-tailed grey-and-white songbird of the southern United
       States able to mimic songs of other birds [syn:
       mockingbird, mocker, Mimus polyglotktos]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "mocker":
   ape, conformist, copier, copycat, copyist, counterfeiter, cuckoo,
   dissembler, dissimulator, echo, echoer, echoist, faker, forger,
   hypocrite, imitator, impersonator, impostor, mime, mimer, mimic,
   mimicker, mockingbird, monkey, parrot, phony, plagiarist,
   poll-parrot, polly, polly-parrot, poseur, sheep, simulator