One of the great "mysteries" of D&D lore that I recall from my youth was a cryptic statement by Gygax in an article of his in Dragon. He was discussing the then-theoretical revision of -- what would become 2e years later -- and he commented that, among the few things he'd be changing in the revised Players Handbook was the weapons table, because he'd inadvertently left out a very common medieval weapon and he wished to rectify this omission. For years, I could never figure out what Gary was alluding to. After all, the 1e PHB includes every polearm under the sun. What could he possibly have left out?
I eventually realized that, in all likelihood -- someone can correct me if Gary said otherwise before his death -- he was referring to firearms. Anyone who's studied medieval military history will know that firearms of various sorts, cannon, and explosives (like the petard of "hoist on your own petard" fame) were widely used in the late Middle Ages. They might not have been as common as they are today, but, from the late 13th century onward, they played an increasingly prominent role in warfare.
For some reason, fantasy roleplaying has generally been down on the inclusion of firearms. Tunnels & Trolls, I recall, included them (as did Blackmoor), but it's always been an outlier in that regard. Firearms are usually seen as the thin end of a technological edge that throws off the equilibrium of many fantasy settings. I rather like the esthetic of archaic firearms and plan to include them in my next campaign. While pondering the subject this week, I had occasion to open a page of Supplement I to OD&D and discovered, on page 14, at the very end of the weapon vs. AC table an entry line for the arquebus. So far as I can find there are no other details of the arquebus anywhere in the official OD&D books (not even Greyhawk), but it emboldened me in my belief that primitive firearms aren't alien to the game.
I'll probably offer my rules and stats for them in an upcoming Grognard's Grimoire.