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.
The animation of Monstro was originally to be assigned to Vladimir Tytla (animator of Doc, Grumpy, Stromboli, and, later, Yen Sid, Chernabog and Dumbo), but, perhaps out of worry that Tytla might get carried away, Disney eventually assigned Wolfgang Reitherman to the task. He animated Monstro as a cunning creature with a brain, making his pursuit of his prey all the more frightening. To Reitherman can also be attributed, to an extent, the timing and staging of Monstro's chase sequences, which were timed to suggest the whale's great weight and power. Reitherman drew a set of sketches for a discarded sequence showing Monstro consuming Geppetto's boat; Monstro was to emerge suddenly on the otherwise serene sea scene, first looking like a large mound before revealing his teeth and swallowing the boat in one movement before disappearing underwater once more.
While Pinocchio spends the night in Stromboli's troupe and later, Pleasure Island, Geppetto searches for his son. Taken to sea, Geppetto, Figaro, and Cleo are swallowed whole (complete with a boat) by Monstro. He later swallows Pinocchio, when the little wooden boy 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.
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 the toymaker took to sea to search for Pinocchio. They are still alive, inside Monstro's belly. Against Jiminy's warnings, Pinocchio resolves to find and rescue Geppetto. Though Jiminy tries to dissuade Pinocchio, warning that Monstro is "a whale of a whale...he swallows whole ships!," he does not hesitate to join 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, in his belly, Geppetto and Figaro are trying to catch fish to eat. The former tells the latter that he fears that they will starve in the belly of the sleeping whale if he doesn't wake up soon.
Monstro wakes at the sight of a school of tuna swimming nearby. 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, though he is nevertheless consumed by Monstro. The sprightly Jiminy escapes his jaws, but, on discovery that Pinocchio has been swallowed, tries to enter Monstro's now closed mouth. But he won't open it. 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 "everything comes in...nothing goes out" of his jaws, Pinocchio suggests that they start a fire to make him sneeze.
Monstro opens his eyes once again, this time to find smoke rising from his blowhole. He begins taking deep breaths; while his mouth is open, Pinocchio, Geppetto, Figaro, and Cleo try to escape and Jiminy 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 then, enraged, he 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 into 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 madly, Pinocchio swims through the hole in the cliff just as Monstro smashes into them, the impact of which sends Pinocchio and Geppetto flying onto the beach. As Geppetto comes to, Jiminy, Figaro, and Cleo are washed onto the shore. Pinocchio, however, didn't survive the blow. He is later revived, and turned into a real boy, by the Blue Fairy. It isn't clear what exactly happened to Monstro, but it is implied that he was killed after ramming into the cliff, however, this remains unknown.
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.
Like in the actual Disneyland park, Monstro is apart of the Storybook Land Canal. At one point in the game, he eats Pinocchio's school books. To recover them, the player must gather some pepper pots to make Monstro 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 on the boats 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 ride's 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.
In the original, instead of Monstro, it was a shark-like sea monster known as the Terrible Shark that swallows Geppetto, and then Pinocchio.
At the time in which Collodi told this story, many mothers would tell their children not to swim too far out to sea or else the terrible shark would swim up and swallow them.
In Collodi's story, the shark is described as being larger than a five story building, a mile long without counting its tail, and sporting three rows of teeth in its mouth that can easily accommodate a train. So fearsome is its reputation, that in chapter XXXIV, it is revealed that the it is nicknamed "The Attila of the Sea".
Unusual for a whale, Monstro is a cross between a sperm whale and a blue whale as he has the body shape and teeth of a sperm whale but has the size, skin color, and pleated underbelly of a blue whale.
Although he is the main villain, his role in the film is small.
According to Richard Ellis, who describe's Monstro in his book, "Monsters of the Sea", Monstro was supposedly so frightening that for many generations, people's opinions were prejudiced against whales; with many believing that whales were dangerous and malevolent. This would later change with studies of real whales in the wild.