When it comes to Ent, I haven't checked yet if the Latin community has been using anything for it, but the easiest thing for the Latin morphology and overall appearance and similarity of the word would be to convert it to a third declension and have
Ēns, entis (genitive case)
.
The problem could be, that this word might have existed in some hypothetical proto-latin (as
ents, entis) and it would mean the participle "being" (not as a noun, but as in "
being good"). Then, on the other hand, perhaps Tolkien could have been inspired by something like the German
der Baum (a tree) which, very hypothetically, could be traced to a related root in the Proto-IndoEuropean that spawned the verb "
be" in English, but I'm not a PIE expert, so perhaps I am being deluded (but Tolkien lived hundred years ago, not a PIE expert either, while a philologist). And then, with some imagination, if you imagine
"der Baum" to have been something like
"a being" (although probably not) and an Ent a giant tree... you can see where I'm going
For an Elf as the elves in Tolkien, usually the community has been using
alfus, alfī...