[fnord123]

>Promethii

Prometheuses.

ii is for latin words. Prometheus is/was Greek. I guess you could use Prometheoí but it would quickly derail any conversation. :)

[rollulus]

There was a talk on Promcon 2016 about this subject [1]. The conclusion was: in English, indeed, Prometheuses. In Ancient Greek: Prometheis.

[1]: https://www.youtube.com/watch?v=B_CDeYrqxjQ

[unixhero]

I kind of like the Ancient Greek version.