Woas said:
What about fey? Are there natural fey predators that live only to destroy the fey?
With fey, it's the Seelie Court versus the Unseelie Court.
Kobolds are noted as having a special hatred for small or tiny fey creatures.
Others include:
Mist mephits versus steam mephits.
Hobgoblins versus elves.
Dwarves versus orcs, goblins, and hobgoblins.
Gnolls versus minotaurs (mainly because the demons they worship are enemies).
Ixitxachitl versus kopru (their patron, Demogorgon, encourages them to war against each other).
Goblins versus kobolds.
Gnomes versus kobolds and goblins.
Orcs versus goblins and hobgoblins (in the afterlife, in particular).
Blink dogs versus displacer beasts.
Beholders versus beholders who look slightly different.
Yuan-ti versus humans.
Sea elves and dolphins versus sahuagin.
Couatls versus rakshasas (in Eberron, anyway).
Couatls versus yuan-ti (makes sense to me).
Devas versus asuras (rivals, not enemies).
Tanar'ri versus baatezu.
Imps versus quasits.
Erinyes versus succubi (redundant, but worth noting).
Lamias versus lammasu (probably).
Griffons versus hippogriffs (really).
Nalfeshnee versus balors (when they can get away with it).
Shadows versus slow shadows (yeah, there's an actual monster called a "slow shadow." I don't know why. Their rivalry with shadows is supposed to be the remnant of an ancient war fought by living armies that continued to fight after they became undead.)
Bariaurs versus armanites.
Snakes versus planes.
Gynosphinxes versus criosphinxes.
Androsphinxes versus hieracosphinxes.
Sphinxes versus manticores?
Treants versus fire elementals.
Halflings versus xvarts.
Monstrous spiders versus spider-eaters.
Warforged versus rust monsters.
Aboleths versus kuo-toas.
Aerial servants versus helions and djinn.
Helions versus efreet, aerial servants, and pyrophors.
Sollux versus efreet.
Undines versus hydraxes, spitters, varrdig, and hordes, as well as watery fiends like hydroloths and wastriliths.
Hydraxes versus anemos, krysts, and undines.
Krysts versus hydraxes and hordes.
Erdeens versus galeb duhr, hydraxes, and hordes.
Dolphins versus sharks.
Thri-kreen versus elves.
Grippli versus grung.
Grell versus... neogi? I don't know.
Werewolves versus wolfweres.