According to Helena P. BLAVATSKY'S The Secret Doctrine, these are half-human entities who were produced by the mating of humans and animals during the Third Root Race.
The anthropoid and other apes were descended from these races. They are the only entities who can enter the human kingdom after the middle point of the 4th Globe of the 4th Round of the Earth Chain, during which point the door to the human kingdom is closed and no new monads can become humans until the next chain.