Star Trek Aliens: The Salt Vampire

This "gender bending" alien could take on the form of either woman or man.  Seen in the show's first episode, The Man Trap, it would do whatever necessary to survive.  This creature sustained itself by draining it's victims of their sodium, hence it's name.  The alien was to have a cameo in J. J. Abrams Star Trek reboot, but the shot scenes were left on the cutting room floor.