>>58712773My theory is that the Decidueye line in Alola has a symbiotic relationship with Dhelmise.
Decidueye gets a chaining move that helps in trap prey just like Dhelmise and in exchange, it spreads Dhelmise's possessed plant matter which Dhelmise needs Decidueye to do because it has no gender and struggles to reproduce in the wild.
This is why in Hisui, Decidueye evolves differently without the Dhemise plant to use for foliage.