The Collaborative International Dictionary of English v.0.48:

Squealer \Squeal"er\ (-[~e]r), n.
   1. One who, or that which, squeals.
      [1913 Webster]

   2. (Zool.)
      (a) The European swift.
      (b) The harlequin duck.
      (c) The American golden plover.
          [1913 Webster]

WordNet (r) 3.0 (2006):

squealer
    n 1: one who reveals confidential information in return for
         money [syn: informer, betrayer, rat, squealer,
         blabber]
    2: domestic swine [syn: hog, pig, grunter, squealer,
       Sus scrofa]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "squealer":
   betrayer, blab, blabber, blabberer, blabbermouth, canary, delator,
   fink, informer, narc, nark, peacher, snitch, snitcher, spy,
   stool pigeon, stoolie, talebearer, tattler, tattletale, telltale,
   tipster, whistle-blower