VGFacts

Naberius is named after the Marquee of hell by the same name or alternatively Cerberus(it is unknown if the greek Cerberus has any connection).

Amduscia is likewise named after the demon King Amdusias.

Lillipa is named after the Kingdom of Lilliput and the lillipans are based on the lilliputians.

Vopar is most likely either named after the Vorpal Sword from the poem "Jabberwocky" in Lewis Carroll's "Through the Looking-Glass, and What Alice Found There" and various adaptations of Alice's adventures in wonderland or the aquatic demon Vepar.

Oracle could be an anagram of Algol similar to Ragol from Phantasy Star Online, but with a c instead of a g.

(if anyone knows what Harukotan is named after, feel free to help out)

Sources:
http://pso.wikia.com/wiki/Phantasy_Star_...t_Naberius
https://en.wikipedia.org/wiki/Naberius
https://en.wikipedia.org/wiki/List_of_de...g_Amdusias
https://en.wikipedia.org/wiki/Gulliver's_Travels
https://en.wikipedia.org/wiki/Jabberwocky
https://translate.google.com/#en/ja/Vorpal
https://translate.google.com/#ja/en/%E3%...1%E3%83%AB (phonetically similar, but pronounced faster).
http://www.pso-world.com/forums/showthread.php?t=207392