Nintendogs: Chihuahua and Friends Unlockables and Secrets (DS)

Nintendo released Nintendogs in multiple versions with minor variations from one another. While all versions of the same language are able to link to one another, each release of Nintendogs features a different set of dog breeds that are available at the beginning of each game.

The Japanese editions each start with five breeds, while the North American and European editions each have six. However, players can earn other breeds during the game but may only "donate" one breed of dog to another owner. This does not mean that the player loses the dog, it only means that the other player/owner unlocks that dog and can purchase it from the kennel. If that owner has a dog that a player wants to unlock and goes on to bark mode with him, then they can unlock that dog as well. (Wikipedia)

Unlockables

Unlockable Dogs
German Shepherd - Collect 17000 Owner Points

Jack Russel Terrier While on a walk, your dog may come across an Item called the Jack Russel Book.
If you visit the kennel, you can now buy the single Jack Russel Terrier

Japanese Style House (from Shiba Pack) - 20000 Owner Points

Pinscher - Collect 14000 Owner Points

Pug - Collect 4000 Owner Points

Schnauzer - Collect 10000 Owner Points

Shetland Sheepdog - Collect 8000 Owner Points

Toy Poodle - 25000 Owner Points

Welsh Corgi - Collect 2000 Owner Points

Unlockable Dogs (US Version)
Once you accumulate the proper amount of Owner Points, new dogs will be added to the Kennel for you to buy and take care of.

Beagle - 16,000 trainer points

Dachshund (dog) - 50,000

Dalmatian - Find an item called "Fireman Hat"

Golden Retriever - Collect 10000 Owner Points

Jack Russel Terrier - While on a walk, your dog may come across an Item called the Jack Russel Book. If you visit the kennel, you can now buy the single Jack Russel Terrier

Labrador Retriever (dog) - 45,000

Miniature Schnauzer - Collect 14000 Owner Points

Pinscher - Collect 2000 Owner Points

Pug (dog) - 35,000

Shiba Inu (dog) - 30,000

Shih Tzu (dog) - 22,000 trainer points

Siberian Husky - Collect 4000 Owner Points

Toy Poodle - Collect 8000 Owner Points

Welsh Corgi - 20,000 trainer points

Unlockable Rooms (US Version)
Once you accumulate the proper amount of Owner Points, new room will be added to the Interior Decorator for you to purchase.

Desktop - Collect 6000 Owner Points

Ranch House - 40,000 Trainer Points

Seaside - Collect 12000 Owner Points

Space - 18,000 trainer points

Urban Living - Earn 25000 trainer points

Easter Eggs

Eat the lightbulb!
When your dog is learning a command, ex: lay down, roll over. A lightbulb icon appears in the top right corner of the screen. Click on the light bulb and record your command. If the dog accepts the command, a sketchy like lightbulb appears over their adorable little head. Grab the lightbulb with your stylus then drag it to the dogs mouth. And they will EAT THE LIGHTBULB like a treat! This also works with bone icons too.

Secrets

Easy way to find hidden presents
When going on a walk, watch the top screen. When the dog icon stops moving, there is a hidden present coming up shortly. Be cautious, however, as this is also true when garbage is coming up, or your pup simply stops by itself.

Get unlockable breeds easier.
Do Bark Mode with a dog in a breed that you don't have.

Example: Do Bark Mode with a Shih Tzu and after you do Bark Mode with that dog, you will be able to buy the Shih Tzu breed at the kennel.

NOTE: This only works the first time with each different version of the game. So you can do this trick one time with Labrador & Friends, Dachshund & Friends, and any other version of Nintendogs.

Here's how you can teach your cute dog a neat song: