Board Thread:Questions and Answers/@comment-5956954-20151221064525/@comment-5498154-20160118145657

Flowey is neither a human nor monster. Since we know that monster souls cannot absorb monster souls, and humans cannot absorb human souls, we can assume that a soulless being is a 'soul vaccum' that can absorb any kind of soul, plus that fact that Flowey is a flower, so there may be special exceptions to the rules considering Flowey is a species that is neither human nor monster.