Where does “ポセイドーン” come from?

ポセイドーン (Japanese) comes from Ancient Greek Ποσειδῶν, from Latin Posīdōn.

Ancestry of “ポセイドーン”, step by step

Step	Language	Word	Meaning
1	Ancient Greek	Ποσειδῶν	Poseidon, god of earthquakes
2	Latin	Posīdōn	—

ポセイドーン — Etymology, Origin & Word History | Etymology Explorer