The Koran or some of its promoters discussed mermaids at some point, therefore they are/were presumed to exist. The question is if you haul in a mermaid along with a nice catch of ordinary fish, is she halaal (permissable to eat)? Can you chop her up, sell her at the market, or take her home to the family for dinner? One source said "all the sea creatures which can only live in water are halaal," so it seems they may be OK to eat for Muslims
http://scienceblogs.com/pharyngula/2009/07/the_mermaid_fatwa.php
Wonder if Twig has any watery fey cousins she should worry about? ::)