There are some “crossover” characters that appear from time to time, as far as I can tell, they’re just in the random pool.
If you download the game, you can go into the files and edit the character stats to make the crossover character more likely or just make the normal characters not show up at all. Just go into the main file, then Cowtastic Cafe_Data, then StreamingAssets. There should be a text file in there that tells you how to edit/create characters.