The Collaborative International Dictionary of English v.0.48:

Ratter \Rat"ter\ (r[a^]t"t[~e]r), n.
   1. One who, or that which, rats, as one who deserts his
      party.
      [1913 Webster]

   2. Anything which catches rats; esp., a dog trained to catch
      rats; a rat terrier. See Terrier.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ratter
    n 1: a disloyal person who betrays or deserts his cause or
         religion or political party or friend etc. [syn:
         deserter, apostate, renegade, turncoat, recreant,
         ratter]
    2: any of several breeds of terrier developed to catch rats
       [syn: rat terrier, ratter]