Hello Ambiversion,
The given names Eulampius and Eulampia are most definitely of Greek origin. Your immediate instinct was correct there! This also goes for the etymology that you provided in your entries for them.
However, these names are not ancient Greek names, but rather, late Greek names. Late Greek names are Greek names that came into being after the advent of Christianity. In turn, ancient Greek names are names that were in use in Greece long before the birth of Christianity. As such, I changed the usage to "Late Greek" in your entries.
To answer your question: if a masculine name is decidedly of Greek origin but yet ends in
-us, the name is
almost always latinized. The amount of Greek masculine names that end in
-us in the Greek language itself is quite small, at least when compared to Greek masculine names ending in
-es,
-ios and
-os. At least, that is what my experience has been so far. Off the top of my head, I can only think of
Odysseus,
Orpheus,
Perseus and
Zeus as examples of Greek masculine names that aren't latinized and actually do end in
-us in the Greek language. Coincidentally (or not?), these all happen to be names from Greek mythology, rather than names that were borne by ordinary ancient Greek men.
Either way, here is a small list of things that in the future will hopefully make it easier for you to determine whether a Greek masculine name was latinized:
*
-aios in Greek is always latinized to
-aeus (which itself will then sometimes be variantly spelled as
-eus). Example:
Aristaios becomes
Aristaeus in Latin.
*
-andros in Greek is always latinized to
-ander. Example:
Alexandros becomes
Alexander in Latin.
*
-aos in Greek is always latinized to
-aus. Example:
Menelaos becomes
Menelaus in Latin.
*
-ios in Greek is always latinized to
-ius. Example:
Theodosios becomes
Theodosius in Latin.
*
-k- in Greek is always latinized to
-c-. Example:
Kleopatra becomes
Cleopatra in Latin.
*
-os in Greek is always latinized to
-us. Example:
Theodoros becomes
Theodorus in Latin.
With that said, if you find that a Greek name has indeed been latinized (no matter if it is an ancient Greek name or a late Greek name), you typically should not list the usage of the latinized form as "Ancient
Roman" or "Late
Roman" respectively. Instead, it should be "Ancient Greek (Latinized)" or "Late Greek (Latinized)". After all, just because the name was latinized by the ancient Romans or late Romans, does not automatically mean that they adopted the name in their language and started using it on their children. Often, when a foreign name was latinized, it was only done in order to make the name easier to understand and pronounce to Latin speakers, the average of which of course never travelled very far from their birthplace and thus was rarely (if at all) exposed to foreign things. The world was smaller back then, not as global as it is today.
But even in today's global world, we still see a similar process: a lot of foreign names (from given names to place names and even country names) are anglicized in order to make them easier to understand and pronounce to anglophones. For example, we say Moscow instead of
Moskva, Netherlands instead of
Nederland and Iceland instead of
Ísland. As for given names, the best examples of this are probably in the 'History' section in the main database: there, you will see many anglicized names, but these were not necessarily used on real people in the English-speaking world. Well, at least not in the time that these names were first anglicized - it may be a different matter in this day and age, where basically anything goes where first names are concerned. "The more unusual or exotic, the better", seems to be the general motto in the English-speaking world now.
Either way, I guess what I am basically trying to say, is: latinized Greek names underwent the same fate back then as the anglicized historical names have (so far). The same goes for latinized Egyptian names, latinized Persian names, and so on. Vice versa applies, too: Latin names were egyptized, hellenized and persianized and afterwards not necessarily used in those languages. No matter what the people, names that were foreign to their language and culture were always adjusted to fit their language and culture better.
So, unless you find strong evidence that a name foreign to a certain language was actually adopted into that language and bestowed upon ethnic people of that language, you should always list the original language in the "Usage" section, accompanied with the language that it was adjusted to. Example: a hellenized ancient Persian name should be listed as "Ancient Persian (Hellenized)". And if still in doubt, you can always look around in the main database and follow the example that is set there. You will see that e.g. latinized and hellenized names are almost always listed by the language of origin.
Alright, that is enough now, I think (I have certainly written more than I intended to, at least!) - I hope you found at least some of it helpful. :)
Proud first-time aunt to Emilia (born November 2015).