The Collaborative International Dictionary of English v.0.48:

Horntail \Horn"tail`\, n. (Zool.)
   Any one of family (Urocerid[ae]) of large hymenopterous
   insects, allied to the sawflies. The larv[ae] bore in the
   wood of trees. So called from the long, stout ovipositors of
   the females.
   [1913 Webster]