Yu-Gi-Oh: Day of the Duelist Easter Egg - Doggy Duelist

When first starting the game (I dont know if this can be done later but its always worth a try) go to your house. There is a dog outside.

Walk up to the dog and start a duel by pressing the R button just like with any other person. The dog has its own personal deck full of dog like monsters.

Also you'll find the dogs name is Nin-Ken Dog.