pocket mouse

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various small nocturnal burrowing rodents of the genera Perognathus and Chaetodipus, found from western North America to northern South America and having fur-lined external cheek pouches, small ears, and a long tail.

from The Century Dictionary.

• noun An American rodent quadruped of the family Saccomyidæ so called from its pockets or external cheekpouches.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of various small nocturnal burrowing desert rodents with cheek pouches and long hind legs and tail