Mule ( a cross between a horse and a donkey) were used as pack animals by early American settlers becauee they are strong, durable, and cheap to buy. Inherant in their characters is a certain freedom of spirit, which means that the animal can take it upon itself to stand its ground and refuse to move, no matter what inducements are used. The means that a mule can be 'stubborn' enough to stand all day, and refuse to move until it decides its point has ben made, and such a stubborn attitude has seen the animal labeled as a stirling example of such behaviour.