dwarf male

noun
  1. a male animal that is much smaller, and often internally simpler, than its female counterpart. Dwarf males are commonly carried by the female, as in species of angler fish
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012