Hatchimals you own can appear in your world, walking around, interacting with decorations and other hatchis and wearing the clothes you dressed them in!
Hatchimals you own will appear in your Hatchtopia, but they need to be assigned to a house.
Place a house in the world or find a vacant house and assign your chosen Hatchimal to the house.
Your Hatchimal will now appear in the world.
See: "How do I assign a Hatchimal to a house?" FAQ for more information.