The Collaborative International Dictionary of English v.0.48:

Pollywog \Pol"ly*wog\, n. (Zool.)
   A polliwig.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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