Yamirei only start to spawn in the latter half of the game.
Yamirei are originally creatures from the Underworld, and spawn from Mother. After Mother consumes Shu Mikami, she is able to release hundreds of Yamirei in order to invade the world so that she can take her revenge against humans.
Yamirei are humanoid, snail-like white creatures, and their only attack is hitting the player with their bodies. They always appear in packs, in dark areas, and that's what makes them dangerous. Like Yamibitos, they can't stand light and won't spawn in a lit area.
According to the myth, Yamirei lived in the surface of the waters before the world was finished. When God created light, the creatures flew down to the deeps of the land and waters, near Yamijima Island. The ones that flew to the depths of the ocean melted in one, forming a terrible, malformed creature called Otoshigo. The ones that flew for the depths of the caves formed Mother, and there they started making plans of taking over the world again.
Apparently, all of the Yamirei originally came from the dead body of a giant skyfish, who fell from the heavens billions of years ago.