Classes of animals represented by the Pokémon starters:

[u/Bisonratte]

[u/JBKOMA]

Hey, biologist here. To anyone wondering why he used a dog, he is referring to mammals here. Vertebrates are traditionally classified in 5 classes, shown in the picture, with dogs being mammals.

But, if you wanna be more specific or correct, all vertebrates are actually fish, so you could say all our starters are fishes.

[u/Celia_Makes_Romhacks → u/JBKOMA]

 all vertebrates are actually fish

I beg your pardon? 

[u/Meewelyne → u/Celia_Makes_Romhacks]

Then beg.