The origin of the rakshasas lies long ago, in the land of Vudra. The human sorcerer Mahka Abihcara, obsessed with his own mortality, desired to continue his enjoyment of material pleasures for all eternity, and so he resolved to live so much that his essence would continue on past his death in a cycle of reincarnation. Mahka became a patron of the arts, a conqueror, and ultimately a cannibal, consuming the bodies of his foes to absorb their life forces. Finally, when he grew too old for his magic to keep him alive any longer, he and his chief generals partook in a feast in which they each devoured a totem animal that, he said, would guide them to their next lives as more powerful beings. Mahka himself consumed a tiger, and ate himself to death.
Twenty years later, a young sorcerer named Purusav Vagbha appeared, seeking out Mahka Abihcara's generals and killing them and their totem animals. Purusav reconquered Mahka's empire, and it soon became clear that he was in fact Mahka's reincarnation, and far more evil and powerful than before. Ultimately, Gopa Citrasena, one of Mahka's generals who rejected his master's evil ways, slew Purusav in an great battle in which it was revealed that Purusav was a tiger-headed monster with backward-bending hands: the first rakshasa.
Unfortunately for Vudra and the world, Mahka and his generals continued to reincarnate, and as Mahka's foul philosophy spread, more evil souls have come into being as rakshasas. The earthbound evils continue to plague Golarion.