What happens to your pet on nintendogs plus cats if you donate it?

Answer 1

If you donate a pet you will not be able to see it ever again. Diego, who runs the hotel, gives you two chances to keep your pet before you eventually donate it. After that you cannot see your pet unless you turn off the game, or your 3DS runs out of power without you saving the game after donating your pet.

There is a chance that, if you go to the kennels, you may see your pet again. Sometimes you can see a pet with the exact same color, markings, gender and personality as your donated pet. This is very rare and sometimes happens when you don't want another pet; don't have enough money or if you reset the game again.