Special effects designers Howard Berger and Greg Nicotero developed 11 different versions of Dren for the film.
Those really are Delphine Chanéac's eyes in her performance as adult Dren. The space between her eyes was digitally widened to create this odd effect.
Vincenzo Natali first got the idea for the film when he saw a picture of a mouse with a human ear growing on its back.
Dren is made out of a combination of human and animal DNA. In particular, the animal DNA that was shown to be used to create her was that of salamander, kangaroo, bird, fish, horse, and stingray.