D&D Monsters: Sahuagin

While mermen have obvious mythological antecedents, the idea of bipedal fish-men such as the sahuagin is a more modern one. This is not to say that occasional pictures resembling such things don't appear in the odd medieval manuscript, or whatever (the Polish bishop-fish is one such example), but there's rarely much detail associated with them beyond the image. Instead, the concept of the sahuagin is more likely to have been influenced by H.P. Lovecraft's Deep Ones and/or the eponymous monster in the 1954 film Creature from the Black Lagoon.

While there are other generally similar-looking fish-men in the D&D canon, the sahuagin have the distinction of being the only one to appear in both the 1st and 5th edition versions of the Monster Manual, so I'll use them as a proxy for the others here. Of note is the fact that, unlike merfolk, they are regarded as "evil" and therefore make suitable monsters, creeping ashore to menace seaside fishing villages or attacking boats.

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1E

As seen in 1E, sahuagin are basically humanoid, but covered in a scaly dark green hide as tough as chain armour. The image shows a somewhat ape-like face with heavy brow-ridges and a spined and webbed crest running from the top of the head down to at least the shoulders and likely further than that. Similarly shaped fins are seen running down the arms and the back of the legs, and there are visible gill-slits on the neck. Physically, they are tougher than typical humans and more intelligent with it.

D&D Monsters: Merfolk

Merfolk are, like centaurs, one of those races that a very common in fantasy settings, but that seem to be present in RPGs largely because they feel like the sort of thing that ought to be, rather than because actually anyone uses them much. Mythically, they are an ancient concept, appearing in tales dating back to the very dawn of civilisation, although, naturally the details vary across time and culture.

Fantasy worlds in literature often include merfolk, although they are often only mentioned in passing. They exist, for example, in both the Narnia and Harry Potter books, and they have become somewhat more popular of late, often in horror films. Some of these versions are inspired by the relatively benign folk of Hans Christie Andersen's The Little Mermaid, while others reflect humanity's ambivalent relationship with the sea, and our perception of fish as 'ugly'. Earlier myths often do a bit of both, with a particular theme being that mermen are ugly and wont to drown sailors, while mermaids are sexy and seductive.