He is much like Moby Dick from Herman Melville's book.
Joe Grant's Character Model Department was responsible for the design of Monstro. Models were constructed both of Monstro himself and of his belly, complete with a skeleton. In addition, inspirational sketch artist Gustaf Tenggren created a watercolor image of Geppetto in Monstro's belly.
As implied by his name, Monstro is a malicious, massive whale with a fearsome reputation among sailors and sea creatures alike, and he isn't like the conventional whale, which is a gentle animal though there are exceptions. He will eat anything foolish enough to wander into his territory and does not seem to care what he consumes, whether it would be fish or humans like Geppetto. He is even said to have swallowed ships whole, or at least destroyed them by using his tail.
Around the seabed where he was sleeping after swallowing Gepetto's boat, there was a collection of sunken ships, though its unknown if the sinkings were caused by Monstro.
Because of this indifference, he is not very intelligent, which Pinocchio exploits by lighting a fire inside him to escape. His obsession with revenge toward Pinocchio and Geppetto is ultimately his downfall, as he smashes his head against a rocky cliff while trying to recapture them.
While Pinocchio spends the night in Stromboli's troupe and later, Pleasure Island, Geppetto searches for him. Taken to sea, Geppetto, Figaro, and Cleo are swallowed whole (complete with a boat) by Monstro. He later swallows Pinocchio, who comes searching for his father. He has a reputation as being a "whale of a whale" and is feared by all the creatures of the sea, and apparently on land, as even Jiminy had heard of him when he tried to warn Pinocchio of the danger involved in saving his father, though he was willing to help.
Monstro is first mentioned in the film when Pinocchio and Jiminy Cricket receive a message from the Blue Fairy who is in the shape of a dove that he has swallowed Geppetto, Figaro, and Cleo when they took to the sea to search for Pinocchio. They are still alive, inside Monstro's belly. Despite Jiminy's warnings, Pinocchio resolves to find and rescue Geppetto. When Jiminy attempts to dissuade Pinocchio, warning that Monstro is "a whale of a whale...he swallows whole ships!" he does not hesitate to accompany him in his search at the bottom of the sea. They search for Monstro; the very mention of his name causes the sea creatures to flee in terror. Meanwhile, inside his belly, Geppetto and Figaro are trying to catch fish to eat. The former tells the latter that he fears that they will starve to death if Monstro does not wake up soon.
Monstro wakes at the sight of a school of tuna swimming nearby. When he sees them, he quickly closes his eyes again and pretends to sleep to lull them into a false sense of security, and there seems to be a smirk on his face. Opening his great eyes, he pursues them with his mouth open, tearing through the ocean. When Pinocchio sees him approaching, he flees for his life, pushing past the tuna, while he himself is consumed by Monstro. The sprightly Jiminy escapes, but, on discovery that Pinocchio has been swallowed, attempts to enter Monstro's now-closed mouth, albeit to no avail. Inside his belly, Pinocchio, reunited with Geppetto, proposes that they escape in a raft (constructed by Geppetto) when Monstro opens his mouth. When told that "he only opens his mouth when he's eating, everything comes in...nothing goes out", Pinocchio suggests that they start a fire to make him sneeze.
Monstro wakes once again, this time to smoke rising from his blowhole. He begins taking deep breaths; while his mouth is open, Pinocchio, Geppetto, Figaro, and Cleo try to escape on the raft, and Jiminy on a green glass bottle tries to follow them; they succeed when Monstro finally sneezes, sending them flying out onto the sea. After another powerful sneeze, he drinks lots of water to put out the fire and then, enraged, pursues the raft while Pinocchio and Geppetto row for their lives. He dives underwater and emerges underneath the raft; Pinocchio and Geppetto row away in time, but when Monstro leaps after them, they are forced to jump into the sea. He smashes the raft to pieces with his tail. Pinocchio saves Geppetto from drowning and pulls him to shore, with Monstro in hot pursuit. As he builds up speed, the waves drifting from the cliff of the shore hinder Pinocchio. Monstro leaps into the air, aiming to consume and kill them. Finally, paddling frantically, Pinocchio swims through the hole in the cliff just as Monstro crashes into it, the impact of which sends Pinocchio and Geppetto flying out and onto the beach. As Geppetto comes to the far end of the shore, Figaro, Cleo, and Jiminy are washed onto the shore. Pinocchio, however, does not survive the blow (though he is later revived and turned into a real boy by the Blue Fairy). It is uncertain what exactly happened to Monstro, but it is implied that he was killed after ramming into the cliff. This remains unknown, however.
Monstro had a guest star appearance in a Bonkers comic story titled "Whale of a Tale", published in the December 1994 issue of Disney Adventures. In this story, he is not villainous, but rather a very polite (albeit destructive due to his size) actor who was simply playing a villainous role in Pinocchio and had not found work in the movies since then. He is duped by a gang of crooks, posing as a movie company, into breaking into banks for them to rob, and upon finding out the truth, helps Bonkers catch the criminals.
Monstro makes a cameo in the episode "Of Mice and Menace".
His Storybook Land Canal Boats poster appears in the background of a scene in Monsters, Inc.
Through archive footage from Pinocchio, Monstro somehow makes his way to take part in the fight at the end of Hockey Homicide.
Monstro made several cameo appearances on House of Mouse. In "Clarabelle's Christmas List", Donald reels him in when trying to reel in Santa's Naughty and Nice list. In "Max's New Car", Pinocchio and Geppetto arrive at the club inside him. As they exit his mouth, Max refers to him as a "Monstro truck". In "Ask Von Drake", he was seen during Ludwig Von Drake's headcount of all the Disney character guests. "Mickey and Minnie's Big Vacation" ends with an advertisement for "Monstro Cruise Lines".
In the DuckTales/Darkwing Duck crossover comic "Dangerous Currency", Monstro has a cameo as the monster form that Dewey takes on as an effect of the slime.
In Once Upon a Time, Monstro is briefly seen chasing Pinocchio and Geppetto. He attacks the raft and leaves.
Kingdom Hearts series
Monstro appears as both a character and world from Kingdom Hearts. Like many of the worlds from the original game, it also appears in Kingdom Hearts: Chain of Memories as well. Here, he is a space whale that swims in between realms. Outside the main area inside his mouth and throat from the film, the rest of his innards are an abstract and colorful realm of purple and green innards.
Monstro can be fought as a boss in Kingdom Hearts Birth by Sleep Final Mix, in an oceanic arena seemingly based upon Pinocchio's world. He appears in Kingdom Hearts as a supporting world, and Kingdom Hearts: Chain of Memories, Kingdom Hearts Birth by Sleep Final Mix as a boss. In Kingdom Hearts 3D: Dream Drop Distance, he appears in his world Prankster's Paradise before it was consumed by darkness, where he directly assists Sora in defeating a Dream Eater boss while Riku navigates inside him.
Kinect Disneyland Adventures
Like in the actual park, Monstro is a part of the Storybook Land Canal. At one point in the game, he eats Pinocchio's schoolbooks. To recover them, the player must gather some pepper pots to make him sneeze. The powerful sneeze releases them.
Monstro's biggest appearance is at the Disneyland version of the Storybook Land Canal Boats. Guests begin the ride with their boats passing through his mouth, which serves as a tunnel to a land of miniature buildings based on various Disney stories. The guides state that the fire Pinocchio made while inside his belly made him sneeze so hard that it blew his tail off, explaining why he has a hole where his fluke should be.
In Pinocchio's Daring Journey, Monstro makes a brief appearance at the climax. Just after escaping from Pleasure Island, the riders are warned by Jiminy that Monstro is approaching. Suddenly, he bursts out of the ocean and lunges at them, accompanied by thunder and lightning. They veer away just in time.
Walt Disney World
Monstro has a spell card known as "Monstro's Water Spout" in the attraction Sorcerers of the Magic Kingdom.
Monstro also appears in both versions of Fantasmic! as a lead-in to Mickey falling into the whirlpool and the stormy ship scene, ending the Dancing Bubbles sequence at the Disney's Hollywood Studios version and coming after the I've Got No Strings scene in the Disneyland version.
- In the original Italian story of Pinocchio, instead of a whale it was a giant shark ("il pesce cane"), which the author had drawn from folklore concerning a terrible phantom shark which mothers warned their children would swim up and swallow them if they swam too far out to sea without supervision. Why Disney chose a whale instead of a giant shark is still uncertain.
- Although Monstro is considered a villain, he could also be considered a threat to The Coachman as Monstro is big enough to eat the coachman's steamboat whole if Monstro was awake during one of the coachman's trips to and/or from Pleasure Island.