The Grass starter, suggested by GamingMints
They are farmer badgers that create their communities by digging through terrain and cultivating food even on themselves. They are Grass/Ground, and are playful and jovial in nature.

The Fire starter, suggested by GlassesSwirly
They are church/cultist bat candles that produce guano out of tears, which is then used as a fuel and to create poisonous crystals on them. They are Fire/Poison, and are skittish and secluded.

The Water starter inspired by joemama6787
They are cold noble newts forming hierarchies down in wet freezing caves. Though they are clever and sly, they are also quite presumptuous and silly. They are Water/Ice.

In case one wondered, the Grass one would be a defense wall, with both attacks at the same and access to extra Psychic moves. Fire would be balanced with high special attack, with extra Ghost moves. Water would be a glass cannon with speed and attacks, with extra Dark moves.

Let me know which one you like and who you'd pick!