The Collaborative International Dictionary of English v.0.48:

Blacktail \Black"tail`\, n. [Black + tail.]
   1. (Zool.) A fish; the ruff or pope.
      [1913 Webster]

   2. (Zool.) The black-tailed deer (Cervus Columbianus or
      Cariacus Columbianus) of California and Oregon; also,
      the mule deer of the Rocky Mountains. See Mule deer.
      [1913 Webster]