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]

The Collaborative International Dictionary of English v.0.48:

Brier \Bri"er\, Briar \Bri"ar\ (br[imac]"[~e]r), n. [OE. brere,
   brer, AS. br[=e]r, br[ae]r; cf. Ir. briar prickle, thorn,
   brier, pin, Gael. preas bush, brier, W. prys, prysg.]
   1. A plant with a slender woody stem bearing stout prickles;
      especially, species of Rosa, Rubus, and Smilax.
      [1913 Webster]

   2. Fig.: Anything sharp or unpleasant to the feelings.
      [1913 Webster]

            The thorns and briers of reproof.     --Cowper.
      [1913 Webster]

   Brier root, the root of the southern Smilax laurifolia
      and Smilax Walteri; -- used for tobacco pipes. See also
      2nd brier.

   Cat brier, Green brier, several species of Smilax
      (Smilax rotundifolia, etc.)

   Sweet brier (Rosa rubiginosa). See Sweetbrier.

   Yellow brier, the Rosa Eglantina.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

catbrier \catbrier\ n.
   a very prickly woody vine of eastern U.S. (Smilax
   rotundifolia) 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, horsebrier, brier, briar, Smilax
        rotundifolia.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Smilax rotundifolia
    n 1: a very prickly woody vine of the eastern United States
         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, horse brier,
         horse-brier, brier, briar, Smilax rotundifolia]