The Jug is a enchanted glass bottle.
The Jug is an extremely sturdy magical container created by the sorcerer Yen Sid to hold Magical Thinner he used in his experiments. The Jug contained a seemingly-infinite amount of Thinner and could resize itself to fit whatever one tried to imprison within it. At some point in the early 20th century, Mickey Mouse accidentally kicked the Jug into the Wasteland through Yen Sid's enchanted map, unleashing a tidal wave of Thinner on the unaware realm.
Years later, Oswald and his allies managed to repair the Jug and make it grow to enormous size. He then placed it atop Mickeyjunk Mountain and lured the Shadow Blot there, trapping him into it (at the cost of his beloved Ortensia's life). The Blot remained imprisoned there until 2010, when Oswald himself accidentally destroyed the cork and freed the monster (though it was soon killed).
Behind the scenes
The Jug was first seen in the 2010 video game Epic Mickey.
Eaerly concepts called for the Jug to genuinely resemble a mountain-sized jug of thinner, as opposed to a mere glass bottle. Rather than house the Shadow Blot, the Jug would initially have been used by Oswald to drain the Thinner Ocean into it thanks to a pump built by the Mad Doctor. Moreover, its inside would actually have been an explorable area, housing none other than Ursula the Sea Witch.
In Epic Mickey: The Graphic Novel, the Jug is depicted as much smaller than in the game, though it is still man-sized as opposed to a regular bottle.