X (updated October 2024)
(social media platform) Once called Twitter, in most cases it should now just be referred to as X.
There is no natural derived noun or verb, as there was with tweet or tweeted. You could use post or posted.
Xbox
(Microsoft鈥檚 rival to PlayStation) ie capped up only at the beginning - and no hyphen.
XL bully
is the dog breed, not bully XL
X-ray
ie upper case "X" - and lower case "r" after a hyphen.