The Collaborative International Dictionary of English v.0.48:

Poitrel \Poi"trel\, n. [OE. poitrel, F. poitrail, fr. L.
   pectorale a breastplate, fr. pectoralis, a. See Pectoral,
   a.] (Anc. Armor)
   The breastplate of the armor of a horse. See Peytrel.
   [Obs.] --Chaucer.
   [1913 Webster]