Had to do some research for this one, but apparently Notch and other Minecraft devs have said they imagine creepers being made out of leaves and other organic material. You know those big piles of raked leaves and compost that start to smoke due the organic material decomposing? I think that’s what a Creeper Pokémon should be based on. It could also be the very first Grass/Fire type.
Patrons can submit prompts for Fake Monday (and Tuesday Chooseday and the webcomic too, by the way) by commenting on the prompt-post I put up every Monday. So if you want to submit your ideas for fake Pokémon, become a Patron!