Melville says that sperm whales can attain a length of ninety feet and Moby Dick was the longest of any of them. However, no sperm whale of quite that length is to be found in the record book. The book is named after the whale as the pursuit of the whale is the theme of the story, even though the creature appears in only three chapters of the work. He is, however, the work's focus.
However, almost as monstrous as the whale himself in a way is Captain Ahab, who pursues him in his ship the Pequod. Ahab does not appear when the ship sets sail, but lurks in his cabin, unseen but present, like some Lovecraftian monster sleeping at the bottom of the sea. When he finally appears he cuts an eerie figure, with a false leg made from the bone of a whale. He is obsessed with Moby Dick and the prime aim of his grim voyage is to slay the mighty cetacean. He makes a speech worthy of Demosthenes in which he enlists the support of the crew for his purpose. He is supported by some shadowy figures whom he has smuggled onto the ship. A Zoroastrian named Fedallah utters dire prophecies about the outcome of the voyage. He hangs about Ahab like a shadow.
While Ahab has no real life prototype, there was a whale on which Melville based Moby Dick. This was an actual fully white whale called Mocha Dick, which lived in the South Pacific. Mocha Dick was generally of tractable bent, often swimming peacefully alongside vessels. Whalers who tried to kill him, however, soon found he could be both ferocious and crafty, generally able to hold his own against them. He was finally killed in 1838. A year later Jeremiah N. Reynolds chronicled his career.