Apple stopped giving their new products the"i" prefix because they weren’t able to trademark it because “you can’t trademark a letter.”
I doubt that they’ll allow Elon Musk to trademark the letter “x”
You can’t trademark a letter and you can’t copyright a font - I believe the courts have ruled so due to first amendment concerns.
I think you can, however, trademark a logo that consists of a letter. I’m thinking specifically of the McDonald’s M logo. I might be wrong, but I have seen Coming to America multiple times.