A Mermaid or Merman with a tail that more closely resembles a marine animal's hind half (curving smoothly like vertebrae in a tail or spine), as opposed to designs that resemble legs wrapped in scales (bent sharply in the middle, like the knees in a leg). Especially if the tail fins are also correct for the type of sea creature (vertical for fish; horizontal for sea mammals).
In addition to the entry above, only use this tag when the merperson's lower body is clearly visible.