Species are created from the Aquapedia. You can see all the Sets and Species you have collected and can tap on a Species to view its information page and Create eggs. Eggs are created using Creation Items like specialised Set Stones or items such as Shells or Scales etc. The items you need for creating a Species are shown on the Aquapedia page and if you need help on where to obtain a specific item, you can tap on one of the items and an information panel will provide more information about this. You can even Create multiple eggs at once provided you have the necessary items, or you can use Aquacash to buy the items you need!