Does Mongoose Gastly count as a mammal? 🤔😂
Have you considered... Mew is maybe the only true mammal in Pokémon?
From dogs to cows, Pokémon in the franchise hatch from a Pokémon Egg. Biologically speaking, the games treat almost all Pokémon as oviparous (egg-laying).
But there is one massive exception in the Gen 1.
If you read the abandoned journals in the Pokémon Mansion on Cinnabar Island in the original Red and Blue games, the text explicitly states:
"Feb. 6. Mew gave birth. We named the newborn Mewtwo."
Mew is the only Pokémon in the entire history of the franchise canonically confirmed to give live birth instead of laying an egg.