The Collaborative International Dictionary of English v.0.48:

Horselaugh \Horse"laugh`\, n.
   A loud, boisterous laugh; a guffaw. --Pope.
   [1913 Webster]

WordNet (r) 3.0 (2006):

horselaugh
    n 1: a loud laugh that sounds like a horse neighing [syn: hee-
         haw, horselaugh, ha-ha, haw-haw]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "horselaugh":
   Homeric laughter, belly laugh, boff, boffola, burst of laughter,
   cachinnation, cackle, chortle, chuckle, convulsion, crow,
   fit of laughter, gales of laughter, giggle, guffaw, ha-ha,
   hearty laugh, hee-haw, hee-hee, hilarity, ho-ho, laugh, laughing,
   laughter, outburst of laughter, peal of laughter, risibility,
   roar of laughter, shout, shout of laughter, shriek, snicker,
   snigger, snort, tee-hee, titter, yuck, yuk-yuk