Solarballs gijinka: Neptunian edition
Neptune has a broad back and lean, toned muscles like swimmers do.
Triton has more of a skinny figure like how mermaids are depicted.
The whole Neptunian orbit are shapeshifters, they keep changing what sea animal they take traits from (some more than others *coff*Triton*coff*).
Neptune and his little moons have dolphin tails (up and down movement) while Triton has a shark tail (side to side movement) to indicate him orbiting in the opposite direction. This could extend to the other captured moons of Neptune or be left as an only Triton thing to emphasize his non-belonging. Perhaps this tail changes after he solves his issues with Neptune?
Before his capture, Triton had a shark tail instead of legs, now it's an orca tail (same family as dolphins, and also called 'killer whale'...). This is because snarks usually live deeper in the sea than dolphins and orcas since they don't need to come up to breathe (aka, he was farther from the Sun before).
Triton has a serpentine tail like how greek river gods were depicted, because "rivers go to die to the sea" and you know what his fate will be...
Neptune didn't originally have a tail, but now uses his shapeshifting to give himself a dolphin tail in addition to his legs. The little moons can actually switch between tail and legs, but prefer the tail to be more like their big brother (who is stuck with only tail instead of legs) and move quicker.
Neptune's little moons like the act like remoras, sticking close to his tail, and he often flings them from it as a game (like how parents would throw their child into the air and catch them, though Neptune just lets them go back to it to fling them again most of the time).
Triton has gills, but the others don't (aka, he is a fish while the others are aquatic mammals). They could be on his chest or on his neck, or the others do have gills but on the part he doesn't.
Webbed fingers. Maybe everyone has them or maybe just some.
Fin-like ears. Like previous.