SQUASH & STRETCH

When creature move, they always change in shape one way or another except for furniture, which is why this principle is important in animation. The best object to demonstrate this would be a rubber ball, the squashing occurs when it hits the ground and the stretching occurs right before it hits the ground and afterwards. In the links they state that a stretch in one direction, must be accompanied by a squash in the other direction. And another important rule to remember is that its volume always remains constant otherwise it will appear off balance.

Sources:

http://www.evl.uic.edu/ralph/508S99/squash.html

http://www.siggraph.org/education/materials/HyperGraph/animation/character_animation/principles/squash_and_stretch.htm