You are talking about summoned beasts. To combine summoned beasts, you need to combine two wild beasts or your own. If you click on the time pet bar, you will see a monster training function. Click on it and a bar will appear for you. If you want to combine the two summoned beasts, just click one. The two summoned beasts you clicked will appear on both sides of the bar. Just click "Learn Monster" again. If you want to give the summoned beast resistance, you just click on the baby you want to fight, then right-click on the item you want to fight and click on the monster.