You get this at the beginning of the game. It is part of the story and cannot be missed. You have to raise three full families which is two adults and two children. The three families are the Sloths, the Beavers, and the Beaky Birdie. You will need to collect gold coins to be able to purchase the companion and the children and you will have to wait for the eggs to hatch. As soon as you complete the last family achievement will unlock.