The Collaborative International Dictionary of English v.0.48:

Horsefish \Horse"fish`\, n. (Zool.)
      (a) The moonfish (Selene setipinnis).
      (b) The sauger.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Moonfish \Moon"fish`\, n. (Zool.)
   (a) An American marine fish (Vomer setipennis); -- called
       also bluntnosed shiner, horsefish, and sunfish.
   (b) A broad, thin, silvery marine fish (Selene vomer); --
       called also lookdown, and silver moonfish.
   (c) The mola. See Sunfish, 1.
       [1913 Webster]

WordNet (r) 3.0 (2006):

horsefish
    n 1: any of several silvery marine fishes with very flat bodies
         [syn: moonfish, Atlantic moonfish, horsefish,
         horsehead, horse-head, dollarfish, Selene
         setapinnis]