There are paintings of bags with horns or flasks attached that date to the 1500s. Until the bombing of WWII, there were bags (dating to the late 1600s) with attached horns or flasks, and paintings of those particular bags in use by known individuals.
There are bags with attached horns in private collections (and probably museums, didn't look) in the UK that were collected 1748-1782 in what is now the US. (There were some pretty neat pouches that didn't have horns attached too.) There is at least one drawing of a Hessian soldier (1780 IIRC) that seems to show the horn attached to his pouch, while others around him seem to be using belt pouches (or possibly cartridge boxes). By 1814 (arguably, possibly earlier--maybe as early as 1803 but this hasn't been shown via documentation), the US Gov't was issuing riflemen a pouch with attached horn.
So there you go. Documented use of a horn or flask attached to the pouch straps . . . if you're re-enacting as a 16th century sailor on a British merchantman landing for water on an unknown island, a German prince hunting in what is now Germany a generation after the Peace of Westphalia, a souvenir-collecting Scottish officer in His Majesty's army serving in the North American colonies during the Seven Years War or the AWI, an oddball Hessian
chasseur serving as an auxiliary to His Majesty's army in Nova Scotia in 1780, or a US Rifleman in 1814-1840ish carrying an 1803 Harpers Ferry/1814 or 1817 Common Rifle/flint Hall breechloader (or possibly in the first year of the US Civil War, when states emptied their armories to arm state troops).