4. Sil (Species)
Sil, played by Natasha Henstridge, appears to be a gorgeous human female at first. However, the truth is far more horrifying. She's actually a human-alien hybrid, created in a laboratory in a DNA splicing experiment. Created to be docile and obediant, Sil actually escapes, rapidly grows in to an adult, and sets out on a mission to mate with a human male - with horrifying consequences. Sil's true form is that of a bipedal alien creature with tentacles, which she turns in to during several of her 'encounters' with men - in one instance, ramming one of her tentacles in to the guy's skull. Certainly not a female you want to involve yourself with...