Amarant is a human, he just happens to have a slightly greenish tint to his colouring. FFIX is full of strange characters that resemble humans, such as Queen Brahne, and whether they are human or not is debatable. The only character who could be classed as 'human', judging from today's norms, is Garnet, but (SPOILER)even she used to have a horn like Eiko's.