4. Sil (Species)
Sil, played by the stunning Natasha Henstridge, appears to be a gorgeous human female at face value. However, the truth is far more horrifying. Shes actually a human-alien hybrid, created in a laboratory in a DNA splicing experiment. Although 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. Sils 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 guys skull. Certainly not a female you want to involve yourself with