Kakuna is an insect in its pupal stage, which wouldn't have a mouth in real life either. Pupitar and Whirlipede are also pupal/chrysalis/cocoon themed, even though they're a dinosaur and a centipede which don't metamorphose in real life. Still, they're mouthless for an explained reason.
Diglett and Dugtrio can be assumed to have mouths under their big noses.
Diglett and Dugtrio can be assumed to have mouths under their big noses.