Bear baiting was a popular sport in Shakespeare's time. A bear was tied by a length of rope to a sturdy post in a pit, unable to escape or defend himself effectively, and then attacked by vicious dogs (or sometimes lions). This is the allusion Macbeth makes as he recognizes that he soon will be overwhelmed by Malcolm's forces:
They have tied me to a stake; I cannot fly,
But bearlike I must fight the course.
Macbeth identifies with the bear. Like the bear, he is hampered by circumstances, but like the bear, he will fight until death. Macbeth's simile suggests strength and courage that is heroic. When Macduff finds Macbeth on the field of battle, Macduff calls him "hell-hound." There is nothing heroic, however, in Macduff's metaphor. Macduff sees Macbeth as a dog from hell, and he intends to send Macbeth back to hell at that very moment.