Wyverns

There's always time to eat, and if the hero is weak, the battle will be over before they can recover. Besides, you don't even need to use them in neutral battles.

Well, there's always time to eat, but the amount of health restored seems to be 20% of the total health of the corpses, which isn't that much, unless the enemy is a large group with a lot of health that's very difficult to defeat. Dark creatures have a level 10 hero with 50 HP and a regeneration of 100, which is half their health. They also have poison, which is a very effective ability.