Big Buck Bunny is the short film's name. It was originally made by the Blender Institute to showcase what the open source software "Blender" could do at the time, since then, Blender has been used to make many different films among many other cool things. That said, not sure it ships with Unity, but might be something that was used by the dev to test video streaming in Unity or at the very least, test how video files get loaded for things like the studio's name that is shown when you launch the game.