The Collaborative International Dictionary of English v.0.48:

Saddleback \Sad"dle*back`\, n.
   1. Anything saddle-backed; esp., a hill or ridge having a
      concave outline at the top.
      [1913 Webster]

   2. (Zool.)
      (a) The harp seal.
      (b) The great blackbacked gull (Larus marinus).
      (c) The larva of a bombycid moth (Empretia stimulea)
          which has a large, bright green, saddle-shaped patch
          of color on the back.
          [1913 Webster]