Unlockables: Chihuahua, Dachshund, and Shiba
Chihuahua: Link to someone playing Chihuahua and Friends
Dachshund: Link to someone playing Dachshund and Friends
Shiba: Link to someone playing Shiba and Friends

Unlockables: More Dogs
Beagle: Collect 2000 Owner Points
Cavalier king Charles Spaniel: Collect 14000 Owner Points
Jack Russel Terrier: When you take your dog for 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
Shetland Sheepdog: Collect 17000 Owner Points
Shih-Tzu: Collect 8000 Owner Points
Toy Poodle: Collect 4000 Owner Points
Welsh Corgi: Collect 10000 Owner Points

Hint: Free Gifts and Toys
While you can purchase new toys for your pup to play with, you can actually get some goodies for free if you know where to look. When you take your Nintendog on a walk, outline a path that goes by the blue "?" boxes marked on the map. Your puppy is likely to run off screen at those points and come back with a present for you!
Your puppy will also randomly run to get presents for you if you leave the game on the "Home" screen for long. If left unattended, your Nintendog will leave the apartment and stay off screen. Simply tap on the center button (the circular one) to whistle and call back your pup who should be carrying a new present.