The Collaborative International Dictionary of English v.0.48:

Trap \Trap\, v. t. [imp. & p. p. Trapped; p. pr. & vb. n.
   Trapping.] [Akin to OE. trappe trappings, and perhaps from
   an Old French word of the same origin as E. drab a kind of
   cloth.]
   To dress with ornaments; to adorn; -- said especially of
   horses.
   [1913 Webster]

         Steeds . . . that trapped were in steel all glittering.
                                                  --Chaucer.
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         To deck his hearse, and trap his tomb-black steed.
                                                  --Spenser.
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         There she found her palfrey trapped
         In purple blazoned with armorial gold.   --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

trapping
    n 1: stable gear consisting of a decorated covering for a horse,
         especially (formerly) for a warhorse [syn: caparison,
         trapping, housing]