The Collaborative International Dictionary of English v.0.48:

horsebrier \horsebrier\ n.
   a very prickly woody vine (Smilax rotundifolia) of eastern
   U. S. growing in tangled masses having tough round stems with
   shiny leathery leaves and small greenish flowers followed by
   clusters of inedible shiny black berries.

   Syn: bullbrier, greenbrier, catbrier, brier, briar, Smilax
        rotundifolia.
        [WordNet 1.5]