Crupper

Our knight did bear no less a pack / Of his own buttocks on his back: / Which now had almost got the upper- / Hand of his head, for want of crupper.

Source: wiktionary

he eſpied a mule's crupper, which hung to the ceiling of the room; this he took down, and tendering it to Don Quixote, went on, ſaying...

Source: wiktionary

Captain went out in the cab all the morning. Harry came in after school to feed me and give me water. In the afternoon I was put into the cab. Jerry took as much pains to see if the collar and bridle fitted comfortably, as if he had been John Manly over again. When the crupper was let out a hole or two, it all fitted well. There was no bearing rein—no curb—nothing but a plain ring snaffle. What a blessing that was!

Source: wiktionary

I sought among the mules one with a mild expression of generosity and gentleness in its eyes, and found it in a white mule with a crupper adorned with arabesques.

Source: wiktionary