Plus, X at the start of a word isn't always pronounced Z.
When I was proofreading the galley pages for my upcoming Enterprise
novel, I tried using the Adobe Reader's speech synthesizer to read the text aloud to help me compare it against my manuscript, and it decided to pronounce Xindi as "shin-dee," as if it were a Chinese or Nahuatl word. (The Xindi do not appear in the book, but are name-dropped.)
Xian (abbreviation for Christian)
Xmas (abbreviation for Christmas, may be pronounced eks-mas)
Although neither of those is actually an X; it's really the Greek letter chi, corresponding to English "ch," and the initial letter of "Christ" in the Greek alphabet. (I've heard some religious types complain about "Xmas" as an abbreviation, believing that it was censoring Christ's name as part of the atheist "war on Christmas" or whatever, but actually it's just his initial in the very alphabet he himself would probably have written it in, given that Greek was the lingua franca
of the Eastern Roman Empire at the time he would've lived.)