The Collaborative International Dictionary of English v.0.48:

Swordtail \Sword"tail`\, n. (Zool.)
   (a) The limulus.
   (b) Any hemipterous insect of the genus Uroxiphus, found
       upon forest trees.
       [1913 Webster]

WordNet (r) 3.0 (2006):

swordtail
    n 1: freshwater fish of Central America having a long swordlike
         tail; popular aquarium fish [syn: swordtail, helleri,
         topminnow, Xyphophorus helleri]