The Collaborative International Dictionary of English v.0.48:

Bullfrog \Bull"frog`\, n. (Zool.)
   A very large species of frog (Rana Catesbiana), found in
   North America; -- so named from its loud bellowing in spring.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bullfrog
    n 1: largest North American frog; highly aquatic with a deep-
         pitched voice [syn: bullfrog, Rana catesbeiana]