Hippo is one of the secret characters that can be unlocked in the Lion King section of Disney Crossy Road! In order to unlock him, all you have to do is unlock the 'Humble Beginnings' set of characters. These characters include Simba, Nala, Rafiki, and Zazu. Once you unlock these characters (in no particular order) you will unlock Hippo!
If you know how to unlock any other Disney Crossy Road Secret Characters, make sure to tell us in the comments below!