Reginald is a perfect example of a P.G. Wodehouse name: so stuffy it's cute. And I love the idea of
Rex as a nn for it. Win-win.
Archilbald is another Wodehouse-esque name.
Ichabod is silly and fun as well, although I get more of a colonial American vibe, as in
Ichabod Crane and the headless horseman of Sleepy Hollow. The potential nn Icky is what keeps me from embracing the name
Ichabod fully.