Please show me where I said that only crusaders have tabards? I’m not sure where you even got this.
I specifically said “crusader tabard” because by this point it’s a trope. I know plenty about the actual crusades and the various knight orders involved. Saying “demon horns” does not mean that only demons have horns. It is instead intended to evoke images of horns associated with demons.
The design is a staple of fantasy at this point. They’re not copying anything but drawing from a collective pool, as most fantasy does with paladins, goblins, etc. and the various iterations of their character designs. Even the colours are not copied, as it has become a trope. It’s a trope that even Diablo itself is drawing on.