What is the little mermaids last name?

Answer 1

It doesn't really say in the movie. Mermaids are different from humans so they may not change their names when they get married. Plus, Mermaids don't really have last names. They have different ways; they don't need last names.

Answer 2

She was the daughter of Triton.

He was supposed to be the son of Poseidon or ( Neptune ) , sister was Ursula. Gods do not traditionally have last names, and there is no evidence that he had one either in the movie or it's spin offs.