Mutation

Mutation sometimes occurs in Pokémon. It can at least cause a Hidden Ability, an Egg Move being learned, or a Hysterical Move being used. However, Egg Moves can sometimes be caused by Wandering Pokémon; this is more likely. Each mutation has a 5% chance of occurring, but there are other intrinsic mutations (Chaos' Eevee has one that increases the chance of it using a Hysterical move by (10 - (current HP/maximum HP))%, for example).