The Collaborative International Dictionary of English v.0.48:

White horse \White horse\
   A large mass of tough sinewy substance in the head of sperm
   whales, just above the upper jaw and extending in streaks
   into the junk above it. It resembles blubber, but contains no
   oil. Also, the part of the head in which it occurs.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

white horse
    n 1: a wave that is blown by the wind so its crest is broken and
         appears white [syn: whitecap, white horse]