I'm not averse to the idea of inventing weaponry, either; as has been pointed out the reality of a D&D world is far different from real Earth history. It's not difficult for me to imagine a weapon that would work better for monster-slaying adventurers than strictly man-on-man fighting. What about something like a short-handled yari (something maybe assegai length?) with a crossguard low enough on the handle to prevent it from working as a versatile weapon? Such a weapon would not be at all suitable for throwing, could be rammed through armor with brute force by STR-based characters or finessed through armor gaps by DEX characters, most definitely would do piercing damage rather than slashing, would definitely be a martial weapon requiring special training unlike a basic spear, and would be a much sturdier weapon than a flimsy rapier for adventuring purposes.