Crossy Road has just been updated with a Christmas/Holiday update. The Update brings 9 new characters including 2 NEW SECRET CHARACTERS. The first secret character is the YETI and the second is the PURPLE BLOOP.
To unlock the YETI, you must be playing as a christmas character. While playing, you have a chance to see the YETI throwing snowballs. If you get hit by one of these snowballs, you will unlock the YETI.
thanks to Piermarco for the info
To unlock PURPLE BLOOP, you must be playing as Grablin. While playing as Grablin, you see Crossy Road characters at the top of the screen with levels next to them. When you defeat a character with a level greater than ~35, you will unlock PURPLE BLOOP Thanks to Top Player for the info