Most languages today, even in countries that have no direct link to Greco-Roman culture, use some variant of the name "Neptune" for the planet. However, in Chinese, Japanese, and Korean, the planet's name was translated as "sea king star" (海王星), because Neptune was the god of the sea. In Mongolian, Neptune is called Dalain Van (Далайн ван), reflecting its namesake god's role as the ruler of the sea. In modern Greek the planet is called Poseidon (Ποσειδώνας, Poseidonas), the Greek counterpart of Neptune. In Hebrew, "Rahab" (רהב), from a Biblical sea monster mentioned in the Book of Psalms, was selected in a vote managed by the Academy of the Hebrew Language in 2009 as the official name for the planet, even though the existing Latin term "Neptun" (נפטון) is commonly used. In Māori, the planet is called Tangaroa, named after the Māori god of the sea. In Nahuatl, the planet is called Tlāloccītlalli, named after the rain god Tlāloc.
What did the Greeks call Neptune? 
Poseidon