You can read more about the peris at Wikipedia.
Allah created the First Man and appointed him the earth for his dwelling-place. And when the First Mortal appeared upon the earth and the peris rejoiced over Allah's wonderful work, the Father of Evil beheld it, and envy overcame his soul. Straightway he conceived a plan whereby to bring to nought that beneficent work, He would implant the deadly seed of sin in this favoured creature of the Almighty, and soon the First Man, all unsuspecting, received on his pure body the damnable spittle of the Evil One, who struck him therewith in the region of the stomach.
But Allah, the all-merciful, the overcomer of all things, hastened to tear out the contaminated flesh, and flung it to the ground. Thus originated the human navel. The piece of flesh, unclean by reason of the Evil One's spittle having defiled it, obtained new life from the dust, and thus, almost simultaneously with man, was the dog created — half from the human body and half from the Devil's spittle.
Thus it is that no Mahometan will harm a dog, though he refuses to tolerate him in his house. The animal's faithfulness is its human inheritance, its wildness and savagery are from the Evil One. In the Orient the dog does not increase, for while the Moslem is its protector, he is at the same time its implacable enemy.