Rubber chicken with a pulley in the middle
From Monkey Island wiki
The rubber chicken with a pulley in the middle is an inventory item first found in The Secret of Monkey Island which was later turned into a running gag in the games' fan community.
In the first game, it was used as a means of transport between Mêlée Island and Hook Island, which were connected by a cable. The item also appears in Escape from Monkey Island, wherein Guybrush briefly finds the item in Grandpa Marley's heirlooms and is given another one (or the same?) by his future self in the Mysts o' Tyme Marshe, and must remember later to give it to his past self at the right time. The item is also mentioned in Tales of Monkey Island Chapter 1: "Launch of the Screaming Narwhal", where Guybrush lures the Marquis De Singe into the Flotsam Jungle, all the while saying to himself, "First, I'll need some dynamite, um, then a rubber chicken with a pulley in the middle, and then some sort of rudimentary lathe..."
Question of Purpose
Part of the item's humour is the question of what its original purpose is. In the original game, Guybrush comes across a Rubber-Chicken-With-A-Pulley-in-the-Middle-Shop, which claims to specialize in "all your rubber chicken with a pulley in the middle needs," though what the item could be used for is not specifically addressed, and Guybrush cannot enter the shop. Upon discovering one of these items across the street in the Voodoo Lady's home, Guybrush comments: "A rubber chicken with a pulley in the middle...what possible use could that have?" The Voodoo Lady herself doesn't seem to have a use for it and lets Guybrush take it without complaining. The item can also be used as an ingredient in the recipe-spell later in the game, apparently substituting for a real chicken.
Non-Monkey Island game appearances
The rubber chicken also appears in another LucasArts game, Indiana Jones and the Infernal Machine, activated with a certain cheat code. In episode three of the first season of Sam and Max ("The Mole, the Mob and the Meatball") a reference to this item is made by Sam when trying to purchase a random item in Bosco's Inconvenience Store.