Mimitchi
Feed a pet that hatched from a black egg only carrots. The pet will transform into Mimitchi after turning six.

Raise a Dog
To raise a dog, feed your pet bread. When it turns six it will morph into a dog!

Secret Mode
With an Adult Tamagotchi, win ANY tournament. You will get a symbol above your tamagotchi's picture on it's stats screen. Now, go to the options menu. There will be a mode called MUSIC MODE. Inside are:

Sound Test
Sound FX test
Sound Test Speed
A gallery of ALL the characters you get after that Tamagotchi wins. They are numbered and you have to get all of them to beat the entire game.

Toilet Train your Tamagotchi
When your Tamagotchi is about 3 years or older, you can press the rubber duck icon(clean)just before they go to the bathroom. A toilet will appear and your Tamagotchi will sit on it.

NOTE: you can tell that your Tamagotchi is going go to the bathroom if he starts to shake.