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.
- 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.
- 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.)
- 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.
- All the characters in Happy Tree Friends are mammals, mostly woodland creatures.
- Subverted in Arthur. Almost all the characters are mammals, but non-mammals have occasionally been shown. They've appeared likely under 5 times in the 20+ years the series has been running, however Funny Animal reptiles and birds have been shown 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, and the episode "Sun Bleached" has a one-shot sea lion character.