Oddly, some creatures which do have an extraplanar origin are considered "natural." These include tieflings, who are descendants of breeding between humans and fiends. Similarly, some creatures exist in other planes but are considered "natural" such as githyanki. Exactly what makes a creature "natural" or not seems to depend on bloodlines then rather than actual location, since drow, which originated on Toril, are considered fey rather than natural creatures. Similarly, githyanki, in spite of having originated in the Astral Sea, are the descendants of humans transformed by illithid enslavery and tieflings, in spite of their outsider heritage, are primarily human in blood.