A character who was able to talk in the original no longer has the mental capability to form and communicate complex ideas in an adaptation.
See, life can be pretty hard for non-human characters. In addition to being treated as more disposable, some higher-ups seem to believe that these sorts of characters reduce the believability of the work. So if they aren't cut wholesale to reduce the effects budget, they tend to be reduced to a non-speaking role.
Talking animals and monsters tend to have an intellect reduced to those of normal beasts, by fiction's standards anyway, while robots and AIs will be reduced to something along the lines of a normal computer. Depending on fan reception of the character in their original form, this will either cause backlash or joy.
- Inverted in the Widget Series Kidou Senshi Gundam San, where some mobile suits from Mobile Suit Gundam are sentient. The Gundam, Big Zam, and Gyan are depicted as females.
- Also inverted in several SD Gundam stories that have Super-Deformed versions of various mobile suits depicted as Mechanical Lifeforms or Ridiculously Human Robots. The Mobile Suit SD Gundam series in particular has human characters from the early Universal Century interacting with sentient versions of the mobile suits they piloted in their original anime.
- Soukou no Strain is an anime based on A Little Princess with characters from other Frances Hodgson Burnett books (Little Lord Fauntleroy and The Secret Garden) appearing as well. Most of the characters have little in common with their book counterparts.
- Ram Dass is an Indian assistant to the rich man who saved Sara from Miss Minchin's abuse in the book. In the anime, Ram-Dass is a Humongous Mecha. In a way, it retains its role as Sara's protector.
- Zigzagged with Emily, who's just a doll in the book. In the anime, the Emily that Sara found is a Mimic, a machine fused with brain cells taken from a Reasoner before birth. To further explain the Technobabble, Reasoners are mecha pilots while Mimics are "keys" for the titular Strains, which are Humongous Mecha biometrically locked to those with Mimics and superior to those mecha that don't require it. The Mimic enables a Psychic Link with the person from whom the brain cells are taken from. A Reasoner who lost their Mimic can no longer pilot a Strain. Sara lost her personal Mimic but, somehow, she was able to use the Emily Mimic to pilot the Ram-Dass Strain. It's then revealed that Emily is a Mimic of an alien race who consisted of pale-skinned identical girls, all of whom share a telepathic connection to one another and the Emily Mimic that Sara found contains the mind of one of two surviving Emilys.
- The Mystery of the Third Planet: Downplayed concerning the indicator. It's not sapient in the original novella, but it has a very high level of intelligence, most notably being an Evil-Detecting Dog towards the diamond turtle and understanding a lot of what goes on during the climactic scenes, so much that one of the villains' mooks mistakes it for a sapient creature and tries to handcuff it. In the movie, however, it only displays the most basic Color Coded Emotions and does nothing plot-relevant.
- In Aladdin (2019), unlike the original movie, Iago does not appear capable of coherent speech (although he's still smarter than a regular parrot)
- Batman & Robin features a rare human example. Bane was a Genius Bruiser in the comics, but in the film he can only communicate in grunts and is implied to be simply too dumb to speak.
- Bedtime Stories has an In-Universe example meant to exploit a literal Life Imitates Art, in a subversion that involves an already normal beast turned into an inanimate object. Skeeter tries to abuse the power of a bedtime story (an alternate interpretation of the words of the story will happen in Real Life) by telling a Western where someone gives his Cowboy Author Avatar a red Ferrari (the horse) for free so that someone in real life would do the same for him but with a red Ferrari car. It doesn't work the way he expected.
- Dora and the Lost City of Gold has it happen to Boots the monkey. Later subverted when Danny Trejo's voice comes out of him in one instance just before the climax.
- Frankenstein: Not only was the Monster in the original novel capable of speech, he was quite eloquent (and more than a bit broody). In the most famous adaptation, the 1931 film, and in most adaptations using that as a launch pad, he becomes a lumbering, thoughtless, non-speaking brute.
- In The Hobbit there are several Talking Animal characters, such as the eagles (especially the Lord of the Eagles), the thrush and the raven Roäc. While these animals all appear in Peter Jackson's The Hobbit film trilogy, they are relegated to relatively small, non-talking roles. The Wargs, who had their own language in the books and were even counted as a separate army at the Battle of Five Armies, also seem to be less sapient and presented as simple beasts ridden by the orcs.
- The Jungle Book's adaptations often remove the power of speech from several animal characters:
- In The Jungle Book (2016), the elephants and the monkeys, who could both talk in both the book and the animated version, show no capability to speak. In the elephants' case it's not to make them seem less intelligent, but rather more mysterious and dignified.
- Mowgli makes the same artistic choice as The Jungle Book (2016), making the elephants and monkeys (plus Rann the kite, who could also talk in the book) The Speechless.
- Marvel Cinematic Universe:
- A Decomposite Character variation with Edwin Jarvis. Due to Age Lift, he is active in the 1950s to the 1970s, and thus doesn't fulfill his comic counterpart's role as The Jeeves to Iron Man and the Avengers. This support role is instead fulfilled by Tony Stark's JARVIS, which was named after him and began as a natural-language user interface but upgraded over time to a more advanced A.I. and eventually uploaded to Vision. As JARVIS appeared in the MCU seven years before Edwin Jarvis did, this trope was played straight for some time.
- Thor: Ragnarok: Fenrir is just a monster serving Hela. He could speak in the original comics and the myths they're based on.
- In Avengers: Infinity War, Black Dwarf (known in the movie as Cull Obsidian) and the Outriders are little more than mindless brutes. Which is a shame in the case of the former, as he was quite the eloquent talker in the comics.
- The Neverending Story: In the first movie, Atreyu's horse Artax is a regular horse and does not speak, while in the original book the two of them conversed.
- In 101 Dalmatians, none of the animals talk. The original cartoon showed dogs and other animals talk to each other, but not to humans.
- In the 1979 film adaption of Salems Lot, the charismatic and cultured vampire Kurt Barlow is changed into a growling Nosferatu clone, with his familiar Richard Straker playing a bigger role, always speaking for him. However, the 2004 TV miniseries stays more faithful to the source material (at least where Barlow is concerned) finally allowing many of his iconic lines from the book to be brought to life (by the great Rutger Hauer no less).
- Salem in Chilling Adventures of Sabrina doesn't speak, unlike his counterpart in Sabrina the Teenage Witch. However, he still seems capable of intelligent thought as Sabrina's familiar.
- Power Rangers
- The Partner Engines that are on par with humans in intellect from Engine Sentai Go-onger are adapted into the nonsentient Zord Attack Vehicles that Dr. K made in Power Rangers RPM.
- When Saban Brands adapted Machalcon, a son of two Partner Engines from Go-onger, into Power Rangers Super Megaforce as Turbo Falcon Zord, it was given a beastly A.I.
- A Decomposite Character example for Barbara Gordon in DC Super Hero Girls. While she is still Batgirl, her Oracle personality (an identity she first took when The Joker crippled her in The Killing Joke) was given to a computer A.I.
- In the original Looney Tunes shorts, Taz the Tazmanian Devil was as sapient as the other Civilized Animals, he was just violent, stupid, and prone to wildly spouting gibberish. In The Looney Tunes Show, Taz is purely animalistic (unlike the other animal characters), and even walks on four legs.
- In Phineas and Ferb: Star Wars (a Fake Crossover between Phineas and Ferb and A New Hope), the ship Centennial Chihuahua stands in for Pinky, Isabella's pet chihuahua (who's also a Deep Cover Agent Partially Civilized Animal) in the regular Phineas and Ferb cartoon.
- Magilla Gorilla in Scooby-Doo and Guess Who? is now a normal gorilla that doesn't speak.
- Snarf in ThunderCats (2011) can now only say his name and is treated more like a pet.