Noun

1. Any horse used in horse-cavalry, but especially one bearing an armored knight.

   As he spoke, the knight-errant, who had remounted his warhorse, galloped forward to the royal stand, with a silken kerchief bound round his wounded arm. — Arthur Conan Doyle, The White Company, Chapter 26.

2. , A regularly revived theatrical or musical work, as with Hamlet or a Beethoven symphony, or as excerpts thereto. May imply that the work in question has become hackneyed.

   2006 Most important though is the fact that, for the first time in I can barely remember how long, the ROH mounts a new production of an Italian repertory warhorse that is fully on the level of the one it replaces [and indeed, in some respects, surpasses it] [...] we actually have a "Tosca". — Google group.

3. An experienced person who has been through many battles, situations or contests; someone who has given long service.

   "Srinath: India's warhorse" (headline from BBC News)