The Collaborative International Dictionary of English v.0.48:

Threadfish \Thread"fish`\, n. (Zool.)
   (a) The cutlass fish.
   (b) A carangoid fish (Caranx gallus, or Caranx crinitus)
       having the anterior rays of the soft dorsal and anal fins
       prolonged in the form of long threads.
       [1913 Webster]