Where's the mouth

When were they numbered

The one in the upper left

Pokémon/Game Freak likes to base off the their characters as closely to it's inspiration as possible. They do it by colors, patterns, it's overall design, it's description in the pokedex. So unless it's based off a frog who eats through its skin that's not gonna happen.

Frogs actually do breathe through their skin

But the mouthless pokemon are usually machine-based or ghosts, not frogs.