Explain possibly unknown creatures or not?

Hi! Probably a stupid question, but here I go all the same.

I'm editing (yet again) the fantasy novel I wrote last year and I have a question about clarifying unknown creatures. Bear in mind it's written in first person.

Do you think it's necessary to explain what a creature is when it's mentioned in the narrative?

E.g, the MC is talking about having read a book and she's listing the elements that appear in it: naiads, bird gods, hidden treasures and romance. No details, just a list.

Not long ago, I got a critique that mentioned that not everyone knows what a naiad is and that it should be clarified. At the time, I added that it's a river nymph, but now that I'm rereading that paragraph, I find it overexplains what should be a simple list.

If it were in a dialogue, I could get a bit more into it, but as is, I think not.

If you think it's not necessary to explain, would you consider doing it if it were a completely made-up creature that'll come up later in the book?