Between Webster's Second International, (over 238,000 words), and the Scrabble dictionary, I only find three:
yex
yox
yunx
xerographically
Windex
jinx
Jinx
Xi
sex, suffix Six
firefox
Nissan 240SX?? lol
ex... EVERYONE KNOWS THAT!
No They Dont Start With "X"
doux means soft in French
No, there are no words in Bridge to Terabithia that start with the letter "x."