Ariel's dad is King Triton, who is the son of Poseidon. That would make Poseidon Ariel's grandfather. Poseidon also has two brothers — Hades and Zeus. According to Disney canon Zeus is Hercules' dad, making Ariel and Hercules cousins.

Did you know that via disney and greek mythology lore The Little Mermaid and Hercules are related

Just gonna yell into the void here with my 2 followers Kevin and BöB