The words are Cantonese.
人
Yan means 'person, human being', cognate of Mandarin
rén.
妖
Yiu is translated, according to an online
Cantonese dictionary, 'supernatural, goblin, ghost', cognate of Mandarin
yāo-- my Mandarin dictionary translates it as (1) 'goblin; demon; evil spirit' (2) 'evil and fraudulent' (3) 'bewitching; coquettish'. The
Chinese Character Dictionary translates it as 'strange, weird, supernatural'.
The latter word is used, for example, in the compound 妖精
yāojing which has two translations: (1) 'evil spirit; demon' (2) 'alluring woman'

Various other compounds using this character mean either 'evil' or 'pretty and coquettish'.
I detect a distinct whiff of
MISOGYNY here. If you do a Google image search for 妖, you get:
http://images.google.com/images?source=ig&hl=en&rlz=1G1GGLQ_ENUS274&=&q=%E5%A6%96&um=1&ie=UTF-8&sa=N&tab=wi lots of cheesecake pics of sexy women but no monsters or demons. Actually, taking a closer look, several of those are pics of Thai transgender women, and the label used on them is 人妖
rén yāo / yan yiu, the term in question. The ever-misogynist Urban Dictionary translates
renyao as 'Chinese insult which means a man without a stick'.
Not only that, the character 妖 is written with the radical 女 meaning 'woman'. I mean, WTF?! Sexist as hell!