They evolve alongside each other, the predator evolves to become faster, better camouflage itself, immunity to the poison of a prey, and likewise the prey evolves to become faster to avoid the predator, camouflage to hide, poison to keep the predator at bay and this cyclic relationship continues with only the strongest and fastest of each species reproducing, just increasing the strengths of both animals.
The predator is almost always larger, and a carnivore. With the prey generally being smaller and herbivore.
