Much like the Unversed, emotions are very important in their designs. Some of the emotions are; Envy, Anger, Sorrow, and Lust. Each of the eyes show what kind the Nightmare is.
Creation and Orgin
The Nightmares were born from the darkness and hatred in Sora and Riku's hearts and minds. They feed off of "bad" or "sinful" emotions and thoughts such as; Sloth, Lust, Anger, and Sorrow. In attempt to rid the universe of them, Xid turned Sora and Kairi into Heartless, not knowing that Kairi is a Princess of Heart. After Kairi hugged Sora, he regained his human form, but was in a coma. This slight mess-up created Namine and Roxas once again. Xid didn't know that Riku was the other source of the Nihgtmares, so she didn't atttack.