Murloc origins are shrouded in mystery. This is due not only to the fact that these creatures appeared on Azeroth's shores fairly recently (as far as world history goes, anyway) but also because murlocs shun mortals and rarely, if ever, speak anything but their own garbled language.
What's been known up until now about the fish-men is the following: they are not the most intelligent creatures. They congregate on shorelines in tribes and villages. They have been known, in certain instances to worship enigmatic sea-deities (sometimes including naga). And they seem to care little for the mortal races.