Ancestral hatred

Each player chooses a creature type, whenever a creature of their chosen type enters the battlefield they may have it fight a creature an opponent controls that has the same creature
type as the one their owner chose.

A few enmities are eternal, notable examples include: squirrels and bidoofs, elves and goblins, wizards and warriors, Boros and Dimir, angels and demons, control and aggro, nerds and jocks, cats and dogs... Alright maybe more than a few.