Thank you qmf, that helped.
Multiple texts that I respect very highly have unhelpful dashes where one might expect to find pariciples of sum; but I find this in Allen-&-Greenough (Section 170):
"The present participle [of sum] which would regularly be sons, appears in the adjective insons, innocent, and in a modified form in absens, praesens. The simple form is sometimes found in late or philosophical Latin as a participle or abstract noun, in the forms ens, being; entia, 'things which are'."
If we take it to be a present participle, that of course explains the -ia in the neuter plural, and the -e in the ablative singular, as in Aquinas' De Ente et Essentia.
Sorry to be confusing, Broken. Let me try to explain:
1. I knew there was a word, ens, which means "being", in the abstract; for one thing it occurs in the title of a work by Thomas Aquinas.
2. I knew there was a word, entia, a plural noun, meaning "entities"; for one thing it occurs in a number of formulations of a philosophical and scientific principal called Occam's Razor.
3. I did not know that these were singular and plural versions of the same word; in fact I "knew" they weren't, since the nominative plural of a neuter noun ens, entis ought not, by the rules, to be entia, but enta.
4. If, however, ens were a present active participle (rather than a noun), its neuter nominative plural would indeed be entia. And despite the fact that multiple authorities say in effect that sum has no present active participle, qmf and Allen-&-Greenough both tell us that in fact it does, and that it's ens!
5. So ens and entia are singular and plural of the same word. Entia, the plural, means "entities". The singular, ens, usually means "being" or "existence" in the abstract. Can it mean "entity" ? According to Lewis-&-Short (a respected dictionary) it can; in fact they define it as "thing".
Therefore, in my opinion, "obscure entity" might best be given as
Ens obscurum.
I would like to thank you, Broken, for raising this question, and you, qmf, for helping to answer it; I learned a great deal from a "simple" two-word translation.