The Collaborative International Dictionary of English v.0.48:

Tadpole \Tad"pole`\, n. [OE. tadde toad (AS. t[=a]die,
   t[=a]dige) + poll; properly, a toad that is or seems all
   head. See Toad, and Poll.]
   1. (Zool.) The young aquatic larva of any amphibian. In this
      stage it breathes by means of external or internal gills,
      is at first destitute of legs, and has a finlike tail.
      Called also polliwig, polliwog, porwiggle, or
      purwiggy.
      [1913 Webster]

   2. (Zool.) The hooded merganser. [Local, U. S.]
      [1913 Webster]

   Tadpole fish. (Zool.) See Forkbeard
      (a) .
          [1913 Webster]

WordNet (r) 3.0 (2006):

tadpole
    n 1: a larval frog or toad [syn: tadpole, polliwog,
         pollywog]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "tadpole":
   amphibian, batrachian, birdling, bullfrog, calf, catling, chick,
   chickling, chicky, colt, croaker, cub, dogie, duckling, fawn,
   fledgling, foal, frog, fryer, gosling, hoppytoad, hoptoad, kid,
   kit, kitten, lamb, lambkin, litter, nest, nestling, newt, paddock,
   piglet, pigling, polliwog, pullet, pup, puppy, salamander, shoat,
   toad, tree frog, tree toad, weaner, whelp, yeanling