Although ethnically Kurdish, the Yazidi have a unique and non-Islamic faith and are considered devil-worshippers by many Muslims.
http://en.wikipedia.org/wiki/YazidiThe origins of Yazidism are ultimately shrouded in Middle Eastern prehistory. Although the Yazidis speak Kurdish, their religion shows strong influence from archaic Levantine and Islamic religions. . . . The religion of the Yazidis is a highly syncretistic one: Sufi influence and imagery can be seen in their religious vocabulary, especially in the terminology of their esoteric literature, but much of the mythology is non-Islamic, and their cosmogonies apparently have many points in common with those of ancient Persian religions. . . .
In the Yazidi worldview, God created the world, which is now in the care of a Heptad of seven Holy Beings, often known as Angels or heft sirr (the Seven Mysteries). Pre-eminent among these is Melek Taus (Tawûsê Melek in Kurdish), the Peacock Angel, who is equated with Satan or Devil by some Muslims. . . . Yazidis believe that Melek Ta’us is not a source of evil or wickedness. They consider him as the leader of the archangels, not a fallen angel, and therefore also comparable to the Christians' St. Michael, who is likewise considered the leader of the "seven who stand before the Lord" and of all other good angels. Also they say that the source of evil is in the heart and spirit of humans themselves, not in Melek Ta’us. . . .
Two key and interrelated features of Yazidism are: a) a preoccupation with religious purity and b) a belief in metempsychosis. The first of these is expressed in the system of caste, the food laws, the traditional preferences for living in Yazidi communities, and the variety of taboos governing many aspects of life. The second is crucial; Yazidis traditionally believe that the Seven Holy Beings are periodically reincarnated in human form, called a koasasa. . . . Auditory resemblance may lie behind the taboo against eating lettuce, whose name koas resembles Kurdish pronunciations of koasasa. . . .
Yazidi are exclusive; clans do not intermarry even with other Kurds and accept no converts. They claim that they are descended only from Adam. The strongest punishment is expulsion, which is also effectively excommunication because the soul of the exiled is forfeit. . . . In her memoir of her service in an intelligence unit of the U.S. Army's 101st Airborne Division in Iraq during 2003 and 2004, Kayla Williams (2005) records being stationed in northern Iraq near the Syrian border in an area inhabited by "Yezidis". The Yezidis were Kurdish-speaking, but did not consider themselves Kurds, and expressed to Williams a fondness for America and Israel.