Mythical merrows were essentially merfolk or nereids, but for the most part (the previously mentioned book The Sea People being an exception), D&D and Pathfinder have used the name for aquatic ogres. Whether they’re actually cousins of ogres or not is up to you—at least one Pathfinder Adventure Path installment (in the Kingmaker series, I believe) used merrows as the result of an amphibious curse instead. Either way, with their extra Stealth in water and their javelins they make good ambushers, grabbing peasant meals or spearfishing for PCs.
A gang of saltwater merrows duels for territory with a gang of scrags. Previously the regenerating scrags had forced the merrows to retreat, but during their last scrap the merrows accidentally discovered the scrags’ vulnerability on land. Just as a party of adventurers reaches a sleepy fishing village at nightfall, the stronger merrows force the scrags up onto the beach for a battle that threatens to reduce the local cottages to flinders.
Merrows are less fractious than ogres—but as creatures of the sea and land they are pulled in many directions, particularly where religion is concerned. Merrows do produce the occasional adept, shaman, druid, or witch doctor, and these spellcasters might heed the call of giant racial deities; deities of the sea, the hunt, or hunger; nature itself; demons; or stranger powers of darkness. When merrows attack outside their traditional hunting grounds or display new tactics, it’s often due to the influence of a new spellcaster and her patron.
On Greenworld, merrows and merfolk are a single species. The beautiful mermaids are the females and merrows are their bestial male counterparts. While their biological drive compels mermaids to mate with merrows, they lust after the comparative beauty and greater intellect of human and elven men. The result is that a cuckolded merrow’s claws have rent many a poor fisherman, and occasionally a young merrow will lurch up from the riverbank, demanding to meet his human father. Even if accepted (obviously a rare occurrence), such reunions almost always end in bloodshed before a fortnight has passed.
—Pathfinder Bestiary 2 189