How to translate the fictional name "yuan-ti" into meaningful Chinese words?
imo, "yuan-ti" is a transliteration of chinese, using older scheme of romanisation. the characters represented are "軟體" (a pliable body), quite a good description of "snake-related-monster".

the "華英萬字典", printed in 1907, 



had the entry of "軟" (juan3, jwan)

https://archive.org/stream/chineseenglishdi00pole#page/339/mode/1up

and, 體 (ti3)

https://archive.org/stream/chineseenglishdi00pole#page/386/mode/1up

it's not far away from "juan" to "yuan" :)