swordtail

sword +‎ tail

swordtail (plural swordtails)

1. One of many species of freshwater fish, in genus Xiphophorus, others of which are called platyfish.
   • 1998 August 21, Gretchen Vogel, “EVOLUTIONARY BIOLOGY:Doubled Genes May Explain Fish Diversity”, in Science‎, volume 281, number 5380, →DOI, pages 1119–1121:

     But ongoing efforts to sequence genes from other fishes, including salmon and swordtail, will also help to test the theory, Postlethwait says.

2. Any of various papilionid butterflies that have a long sword-like projection from the tornal section of each hindwing.

• five-bar swordtail
• green swordtail
• red swordtail