Domestication of wild creatures provides defense and also eggs during the Tribal Stage of the game; the eggs can be eaten by herbivores, omnivores and carnivores. It costs fifteen food to feed a wild animal and thus domesticate it.
If you advance to the next Tribal Stage with creatures that are not your own species in your pack, these creatures will become pets/domesticated animals in the Tribal Stage. These pets are much smaller than creatures domesticated in the Tribal Stage, but they provide eggs in the same way. If another tribe attacks your tribe, the domesticated animals will fight, the tougher animals (such as rogues that became pack members in the Creature Stage) take a huge toll on the enemy. Pets can also fight when they are left alone, though can't see animals stealing food. Like all pets, they can get mad. Make sure you don't leave any food behind. Do not use the Refreshing Storm, Flying Fish or Traps special abilities nearby. If your species attacks their species, they can become hostile to your tribe, but the pet will stay as a mad pet. They won't attack the other pets in their spot. If they have the red mad face, they cannot be befriended again. A scared face gives you another chance to ally with their species, though.
Occasionally the pets will get abducted by a passing UFO, usually 2, though other times it will be one and the second time the UFO will abduct a tribal member from another tribe. The simplest way to get around this is to domesticate 'replacement' animals. A tip: If you want food, keep a nearby nest of creatures for available domestication and even protection from other tribes (Beastmaster consequence ability, and hostility). Another tip is to domesticate three animals and, after a set amount of time, send one tribesman to gather eggs for a steady food income that can sustain you through the entire stage.
Pets will defend your tribal area. If an enemy tribe sends raiders to attack, and the raiders arrive near the pet pen, then your pets will attack the raiders. This can prove useful as Rogues can generally defeat tribesmen, or problematic if you want to socialize (you will have to use fireworks to get them to stop attacking so that you can socialize).