Vookie is a virtual cow that lives in the real world by proxy of this application. Simply download, print, and attach the image to a wall or other vertical surface. Then, with the app running, point the camera toward the picture. (Works best if the whole image is in view of the camera while taking up most of the view).
Vookie: AR Cow is an augmented-reality toy app, not really a game.
The AR image can be downloaded from here: http://i.imgur.com/QQzPrz6.png
For best results, print the image such that it the width is around 4 inches.
Also includes a cardboard VR mode.