It was probably a ‘th” at one time based both on how the Temanim pronounce it, and more importantly, on how the Talmud transliterates words from other languages. All languages tend to evolve over time, which is why a lot of English words are “mispronounced”, for example not saying the final “e” in “come”, and totally mauling words such as “light” (which should be prounced the same as the Yiddish “licht”).
