I love
Rex as a person name, it's eccentric and fun. It's funny that so many people think of it as a dog name, when I don't think
Rex has been a common dog name for something like 60 years? I think it's useable.
It could be a unique nickname for something like
Reggie, instead of
Reg. Especially if it was "
Reginald Xavier" or something. Oh,
Reginald was actually the real name of the actor
Rex Harrison! So it's not too much of a stretch. Or maybe
Richard, because that's a "kingly" name? I actually like that a lot.