saddlebag

Short definition from Wiktionary

Noun

1. 1
   A covered pouch, usually one of a pair, laid across the back of a horse, donkey, or mule behind its saddle, or hanging over the rear wheel of a motorcycle; often made of leather or (on a motorcycle) a rigid material.
2. 2
   pannier
3. 3
   A seat pack or a seat bag (a pouch or a small bag attached to the seat or seat post of a bicycle)
4. 4
   Loose fatty flesh on a person's upper thighs or buttocks, that hangs like saddlebags.
5. 5
   A style of house with two rooms separated by a small hall and open space.

Verb

1. 1
   To place in a saddlebag.
2. 2
   To put a saddlebag on (an animal).