O.K., cool. Well, mermaids have their own language; they're really smart though, and quickly learn other languages and stuff. Their tails are fragile although composed pretty much of layers and layers of thick muscle; the tail is a continuation of the spine until the mermaids change to human form. Their hair has a shimmery off-colored look, even when dry, so they don't look 100% human. When I say "shimmery", I mean that mermaids' black hair has a hue like sea glass and blond has a light blue tint when they move, etc. They also have very sharp features.
The mermaids were probably created during Classic Antiquity, though they may be much older than that. Nothing is known about the Ancients' reasons for creating mer-people; but as details emerged in the late Sixth Age about the Roads War, some historians speculated that they may have been created as aquatic super-soldiers. Secondary sources suggest (albeit alliteratively) that the Ancients were not above genetic manipulation to gain an edge.
Though known vaguely to the Alliance, the existence of mermaids was not confirmed by the Gudersnipe Foundation until late in the Sixth Age. The planets they are found on, are in mostly remote autonomous regions of Runarin space.
Mermaids never developed space travel (or even electricity), so their presence on multiple planets can only be explained by the Ancients.