Chespin is a shrew and Chesnaught is an armadillo because of evolution. See, All grass-type starters are based off of creatures that lived many millions of years ago. Bulbasaur is a dicynodont from the time before dinosaurs. It was a synapsid, which is an animal resembling a mammal and reptile. Meganium was a sauropod, which appeared early in the dinosaur eras. Sceptile is a dilophosaur, which lived sorta in the middle. Torterra an ankylosaur, which was mid-late. Snivy and Serperior being the small lizards that evolved into snakes between the time of the dinosaurs' end to the beginning of mammals. Chespin is the first mammal, a shrew, which first evolved into armadillos and later rabbits, monkeys, and so on. Fire-types are from the Chinese Zodiak (Dragon, Mouse, Rooster, Monkey, Pig, and Dog). Water-types are simply just amphibious.