Bag
air reservoir: flexible (usually of animal skin), ready for the insertion of stocks
Flexible reservoir made of airtight material: processed animal skin (usually with the shorn hair on the inside; NB: ➺ below), tanned leather, or some synthetic fabric, or rubber (inner car tube); Often covered with a decorative cloth.
The head, hind, and all legs are cut off, the necessary stocks for pipes inserted and tied into neck and forelegs, and the hind tied up. In several regions in eastern Europe, North Africa, and the Middle East, the "tanning" is rather rudimentary (i.e. with a saline solution, which ensures tightening of the pores), but effective.