>>54103481A Digimon raised in a pen filled with shit. Shit is its environment. It's never given medicine, so it must evolve to be immune to shitty diseases. It's unfed? Well it evolves to be able to safely eat shit, too. It's never bandaged while injured? Well let's evolve to get rid of bones and anything else that could go wrong, becoming invertebrate.
Even trash mons become who they are to be more acclimated to the environment and conditions they've been in.