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.
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 Geppetto's boat, there was a collection of sunken ships, though it is unknown if the sinkings were caused by Monstro.
Because of his indifference to his prey, he is not very intelligent, which Pinocchio exploits by lighting a fire inside him to escape. He does have a degree of intelligence though, as he pretended to be asleep upon seeing a school of tuna in order to lure them closer before attacking, and had the sense to use water to put out the fire making him sneeze, and upon noticing Geppetto and Pinocchio's raft, instantly knew they were responsible for the fire.
He also has a vicious temper, as Geppetto told Pinocchio that his plan to make him sneeze would make him furious. Indeed, he relentlessly pursued and attempted to kill them over it.
Ultimately, however, his obsession with revenge toward Pinocchio and Geppetto was what lead to 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 in the form of a dove that he has swallowed Geppetto, Figaro, and Cleo when they went to 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 creates a violent wave that 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".
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 Once Upon a Time, Monstro is briefly seen chasing Pinocchio and Geppetto. He attacks the raft and leaves.
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.
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.
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.
- Monstro is the only antagonist in Pinocchio to receive an on-screen comeuppance; in this case crashing into the cliffs and possibly being killed.
- In the original story, instead of Monstro, a shark known as the Terrible Dog-Fish (il Terrible Pesce-cane in original text) swallows Geppetto and Pinocchio. Instead of chasing them and crashing onto rocks, the Shark simply never knew they escaped, since they went out when he was sleeping with his mouth open in surface.
- 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 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 it is nicknamed "The Attila of the Sea".
- When Pinocchio makes the smoke, Monstro sneezes before he finds out that his mouth is on fire, but technically the blowhole is a whale's nose.
- Monstro is referenced in Mickey, Donald, Goofy: The Three Musketeers, when one of the Beagle Boys made a jest at Pete.
- According to Richard Ellis, who describes Monstro in his book, Monsters of the Sea, he 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.
- His name might be based on the Portuguese and Spanish words for "monster", monstro, or it might be taken from the English word monstrous, which can mean "gigantic" or "enormous" depending on the context.
- His design had been reused on multiple occasions in Disney franchise, such as in the Aladdin TV series (for example, in "When Chaos Comes Calling" and as one of magical creatures seen with Genie in which the latter was as Moby Dick).
- Monstro is the first Disney character voiced by Thurl Ravenscroft.
- The fact that Geppetto is living inside Monstro is a reference to the story of Jonah, in which when Jonah is swallowed by the whale/giant fish for 3 days, the two different humans survive in the belly of the two different sea creatures.
- Monstro appears to be a hybrid between a blue whale and a sperm whale. He has the size and underbelly of a blue whale, but the head and teeth of a sperm whale.
- Monstro's colour changes between the media in which he appears: sometimes it's black, sometimes various shades of blue, sometimes even grey.
- The Kingdom Hearts series is the only instance in which Monstro's teeth are squared and not pointed.