Minecraft wiki entry for Monster_infighting, associated text: Wolves attack skeletons, skeleton horsemen, spider jockeys, wither skeletons, sheep, and foxes unprovoked.
Guardians and elder guardians attack squid.
Llamas attack wolves.
Polar bears attack foxes.
Foxes attack chickens and chicken jockeys.
Trader llamas attack any mob that attacks their wandering trader.
All types of illagers and zombies (with one exception) attack all types of villagers.
Ocelots attack chickens.
Cats attack rabbits.
Endermen attack endermites.
Piglins occasionally hunt hoglins.
Piglins and wither skeletons attack each other.
Wither Skeletons attack baby turtles, iron golems, snow golems, piglins, and piglin brutes
Withers attack all mobs except undead mobs and ghasts.
Iron golems attack most hostile mobs
Snow golems attack most hostile mobs.
Zoglins attack most mobs except creepers, ghasts and themselves.
Vindicators named "Johnny" or have the Johnny tag set to true attack most mobs except illagers.
Axolotls attack drowned, all types of fish and squid, guardians, and elder guardians.
Undead mobs (with two exceptions) attack baby turtles.
Zombie variants and blazes attack turtle eggs.
Witches will sometimes attack one another.