Is the human race the only one originated on Eternia? All other "freaks" come from different dimensions or planets (Skeletor, Hordak, Beastman, Merman, Webster and so on...).
It seems to me weird Eternia brought so much biological diversity to its living spectrum. That's my guess...
Beast Man, Mer-Man and Webstor all came from Eternia, but to my knowledge, it has never been established how their ancestors came to be there, or if they just evolved there. Very interesting topic.