The Collaborative International Dictionary of English v.0.48:

Whiffletree \Whif"fle*tree`\, n.
   Same as Whippletree.
   [1913 Webster]

WordNet (r) 3.0 (2006):

whiffletree
    n 1: a crossbar that is attached to the traces of a draft horse
         and to the vehicle or implement that the horse is pulling
         [syn: whiffletree, whippletree, swingletree]