When you start your game of HM3, you have 8 pets you can choose from. There are 3 dogs, 3 cats, a bird, and a pig. The pets aren't just for looks though; each one has a specific job that they help you do on the farm.

Big Dog
Big Dog
Protects your animals. If you leave your livestock outside over night with the dog outside as well, the next morning they won't be grumpy. There are some things you have to remember though; there has to be mature grass for the animals to eat while they're outside and it can't be raining.
Medium Dog
Medium Dog
Same as the Big Dog.
Small Dog
Tiny Dog
Same as the Big Dog, but a lot cuter.
Pig
Pig
Will bring you little bits of money in the morning.
Pink Cat
Pink Cat
The white cat on the selection screen, appears as pink in the game. This cat will randomly find you Spring seeds.
Gray Cat
Gray Cat
The black cat on the selection screen, but it's gray in the game. Will bring you random Summer seeds.
Brown Cat
Brown Cat
Appears as the spotted cat in the selection screen. Will find you random Fall season seeds.
Bird
Bird
Brings home any season's seeds.

If you have a pet that brings you something you'll never know when they will do so. You will receive the item they collect in the morning when you wake up, but you won't know if you have it unless you check. The Pig brings home small amounts of money so the night before you'll need to know how much G you actually have and then compare that to what you have when you wake up. The cats and Bird bring seeds so just check your seed inventory for anything new. The cats will only bring you seeds when you're in the season they're assocated with. A Gray (Summer) Cat will only bring you seeds in the Spring, the Pink (Spring) Cat brings seeds in Summer, and the Brown (Fall) cat will find seeds when you're in the Fall. The Bird will bring you seeds in the Spring, Summer, and Fall since it's not season-specific.

The pets will only be of some help to you if they're left outside over night! Leaving your pet inside will prevent it from doing its bonus job. Your pet won't mind the rain at all!