For me it is like saying, some made a fictional invention lets say that you can tast something from a distance. And calls it teletaster. And then someone saying "Ow !!! they stole/copied that from the television !!" the word tele has been used as a prefix for so many things, no one would call using tele stealing, not just because it has been used often, but because it means what it describes. "far" something far you use tele for, something snake like you use naga. Just check computer names, many snake like monsters have names with naga (or asp) in them. (you can learn a lot from game names, the devoloper usually dont just pick names but hail back on mythology or ancient history.It has a "downsde" though, many kids know names just from that instead of the actual thing).
If anything you could say Kipling stole it... the word nagini allready literally excisted. You can research the root naga and all the places it has been used in and mytholgies around it for hours yourself. But here is an interesting article which shows a part of it
Edit: sorry missed marlowe comment. same link ;) but it is not the source, the word is much older. I mean the word isnt taken from the myth (though kipling might have taken the word directly from it yes, if that is what you meant by source, not source of the word itself, but source of kiplings use). but yes one of the ways/place a words with the root naga is used.
Weird though how low the scores is, I thought everyone knew a snake had a split tongue, or maybe they didnt know what bifurcated is? I knew and english isnt my language and we dont have a word that resembles it (just a word resembling split or spliced) and it is not a romance language either. I think the word is easy to remember. Everybody ( would hope) knows the prefix bi- and furcated looks like forked. And that is actually what it means (and where it comes from so not just a mnemonic) It is forked (in two).