The Starter Egg is a free egg that is given to you when you enter Adopt Me! for the first time and go to the Nursery where Sir Woofington is. You can only obtain a Dog or a Cat from this egg. You cannot obtain it anywhere else in the game and it cannot be traded. You can only get one of these when you first play Adopt Me!. It is the first egg that has a 100% chance of hatching into a common pet, with the second one being an Easter 2020 Egg. You also have to be an adult to get the egg.
Note: It is sometimes stated that the Buffalo hatches from the Starter Egg, but it does not.
The only pets you can get from the Starter Egg are a Dog or a Cat. This egg cannot hatch anything else.