The Collaborative International Dictionary of English v.0.48:

Watchdog \Watch"dog`\ (w[o^]ch"d[o^]g`), n.
   A dog kept to watch and guard premises or property, and to
   give notice of the approach of intruders.
   [1913 Webster]

WordNet (r) 3.0 (2006):

watchdog
    n 1: a guardian or defender against theft or illegal practices
         or waste; "she is the global watchdog for human rights
         abuses"
    2: a dog trained to guard property [syn: watchdog, guard
       dog]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "watchdog":
   Cerberus, Seeing Eye dog, bandog, bitch, bowwow, canine, cerberus,
   clanger, custos, dog, fancy dog, guard dog, guardian, guide dog,
   gyp, keeper, kennel, lap dog, pack of dogs, pooch, pup, puppy,
   sheep dog, show dog, sled dog, slut, toy dog, warden, whelp,
   working dog