The Collaborative International Dictionary of English v.0.48:

Destrer \Des*trer"\, Dextrer \Dex"trer\, n. [OF. destrier, fr.
   L. dextra on the right side. The squire led his master's
   horse beside him, on his right hand. Skeat.]
   A war horse. [Obs.] --Chaucer.
   [1913 Webster]