In Real Life, there are many species that theorists have speculated can evolve over time into something of comparable sentience and intelligence to humans. Not all of them are mammals.
In fiction though, many works exist in universes full of nothing but humanoid animals or humans interacting with humanoid animals. Many of these works feature humanoid animals of many types, but often focus exclusively on mammals. More specifically land-based mammals, as rarely are exclusively aquatic mammals, such as dolphins or manatees, ever included.
This trope can occur for many reasons. The designers might find humanoid reptiles or birds too weird looking or hard to draw. Or possibly, they also being mammals, just prefer mammalian animals or are more familiar with how mammals work. Another reason for this might be a way to avert Carnivore Confusion. Instead of having Vegetarian Carnivores or having characters eat meat without explanation as to where it's from, instead it's implied (or outright shown) that they eat non-anthropomorphic birds or reptiles.
This trope is sometimes subverted by throwing a token bird or reptile into the mix.
- Coco Pops mascot lives in a world where there are animals in a jungle or a city with only mammals in it with birds and reptiles.
- Maple Town features Funny Animal rabbits, bears, dogs, and foxes but no fish or birds.
- In One Piece, the Minkmen are all Beast Men who are based on only mammals. All of them, including the ones based on herbivore mammals, can and do eat meat (usually that of reptilians or fishes).
- In Seton Academy: Join the Pack!, every student shown is a mammal, whether a Little Bit Beastly girl or an upright realistic-looking male. However, the teachers are dinosaurs (well, most of them are dinosaurs with one pterosaur and one spider-monkey), while the headmaster is a prehistoric anthropod called an Anomalocaris.
- Averted in Maus; the central characters are depicted as mice and cats, but the animals depicted also include frogs, fish and moths.
- Zootopia is a city populated by evolved mammals set in a world where humans never existed. Therefore, animal species domesticated by humans (such as domestic dogs) rarely appear, although there are a handful of exceptions. (For example: domestic sheep, horses, and pigs exist) Word of God mentioned that primates were not shown in the movie because they appeared too human-like (but didn't deny their existence, and concept art for the movie features a bushbaby), and has hinted, but not confirmed, that a city inhabited by reptiles and birds may exist. The only nonmammals seen at all are the (nonanthroopmorphic) houseflies flying around Yaxs head, and the implications of a restaurant called Bug Burger.
- Subverted in Rock Dog. Initially it seems to take place in one of these. It features mammals such as dogs, foxes, goats, cats, sheep, and wolves. However, reptiles such as crocodiles also show up.
- In Cat City, only cats, mice, rats and vampire bats are portrayed as anthropomorphic, whereas caimans, snakes and parrots are Nearly Normal Animals (although the parrot can talk). The sequel throws an anthropomorphic badger into the mix (except it doesn't, the badger in question is a mouse in disguise), but also shows a non-anthropomorphic leopard, sloth and buffaloes.
- Inverted in The Good Dinosaur, where dinosaurs (at least non-avian ones) and pterosaurs are sapient Partially Civilized Animals, but mammals (including humans) and birds are Nearly Normal Animals.
- Subverted by Robin Hood; the majority of the cast are mammals (including foxes, lions, bears, wolves, rabbits, mice, badgers...), but there are a few token non-mammals, like Sir Hiss and Lady Cluck (who are, respectively, a snake and a hen.)
- Tasakeru is somewhat like a Darker and Edgier take on Zootopia above. Humans never existed, and selected mammal species have evolved to become anthropomorphic "sentients". Unlike Zootopia, the sentients are shown to have domesticated animals such as chickens, goats, sheep, and pigboars.
- Barsk The Elephants Graveyard takes place in a Milky Way populated by over eighty species of anthropomorphic mammals, and only mammals. This turns out to be because they were all genetically engineered by humans to be capable of language, and only mammals had compatible physiology.
- A majority of the Sonic the Hedgehog cast are mammals, even though Team Chaotix, which consists of two reptiles and one insect, and the Babylon Rogues, a team of birds, are thrown in.
- Inverted with Splatoon, where everyone is a marine invertebrate whom some can transform into human-like forms. The only mammal, besides his clone, is Judd, a cat whose Last of His Kind.
- The two races in TailConcerto and Solatorobo are anthropomorphic dogs and cats.
- All the characters in Happy Tree Friends are mammals, mostly woodland creatures. The show only has ever had one non-mammalian character with Sneaky, who only ever appears in the W.A.R. Journal episode of the Ka-Pow spin-off.
- Precocious Exaggerates this trope, by having only canines and felines be civilized. This was done as a means of preventing Furry Confusion.
- AGENCY is a Massive Multiplayer Crossover webcomic where most of the characters are Funny Animal mammals from various pre-existing works. The only exception is Pikachu, who is treated as a "normal" mouse by the Funny Animal characters.
- Downplayed in Arthur. Almost all the characters are mammals, but non-mammals very occasionally show up. Most often, Funny Animal reptiles and birds appear in the background or as gags.
- Inverted in SpongeBob SquarePants. Almost all of the characters are invertebrates or fish, with the only recurring mammal characters being Sandy (a squirrel in a diving suit) and Pearl (a whale). Other whales have appeared from time to time as background characters, the episode "Sun Bleached" has a one-shot seal character, and Kenny the Cat appears in his eponymous episode..
- Inverted in Amphibia, where the only animals that are sapient and anthropomorphic are amphibians such as frogs, toads and newts. The few mammals that appear other than our Token Human lead (such as hedgehogs and weasels) are non-sapient and often monstrous.
- Timothy Goes to School plays this straight. All of the characters are mammals, while other animals seem to be the same as in real life. For example, Nora (a mouse) has a pet lizard.
- Subverted in both versions of DuckTales. Half of the world's populace are birds, while the other half are mammals. Anthropomorphic reptiles, amphibians, and invertebrates are shown to exist, but only as supernatural beings.