So are all different longswords. They vary in size, shape, and style. It's a club, as is a baseball bat, a tree branch, or a chicken-legged shaped thing.
What aspects about it do you feel are notably different to all of those things?
why should a Katana be different than a bastard sword if they function the same
The Blackthorn is an Irish "walking stick" which was invented when the British outlawed theshillelagh. I know the shillelagh is a spell in pathfinder, but I'm referring to the actual weapon used and revered in Ireland. What stats and proficiencies should I give it?
It shouldn't, really, IMO.