10. Sil - Species (1995)
When Species debuted in 1995 its popularity was largely due to the appearance of blonde bombshell Natasha Henstridge as alien-human hybrid Sil, who spent most of the film running around in the buff and seducing men to their deaths. Sure, Henstridge was the sexiest movie monster since Mathilda May's space vampire in Life Force, but nude debutante isn't going over well with Human Resources at the office Halloween party and it's likely to get you arrested or down with pneumonia if you're out on the Trick or Treat beat. Maybe that's why Sil never really caught on as a costume, but there's a simple and easy solution for those wanting to traipse about as a unique villainess this Halloween. As the film ends, Sil sheds her buxom façade for an H.R. Giger crafted monster that looks like Alien's Xenomorph and Metropolis' Dark Maria got together and produced offspring. She's a daunting design but the right combination of bodysuit, cardboard, foam and spray paint could result in a startling and stunning costume that's still sexy in its own strange way.