Captain Ahab's peg leg is made of whalebone ivory. Ahab lost that leg when he was attacked by the White Whale, Moby-Dick, during a voyage that took place prior to the events of the Herman Melville novel "Moby-Dick," which was first published in 1851.
Ahab's loss of limb is an important element in his character; it fills him with a desire for revenge against the White Whale. It is this desire that drives much of the novel's plot. Melville describes Ahab as a monomaniac, singly and obsessively devoted to taking revenge on Moby-Dick. Ahab's hatred for the whale and his desire for revenge becomes so strong that he blames the whale for more than just taking his leg, seeing the whale as a grander force of evil and forcing his crew to help him seek revenge.