Can pets go to heaven or is it only believers (i.e. humans)?

No.

Heavens are strictly for believers of the right cult.

If your brain correctly believes the correct way you will somehow end up in heaven.

If your brain sadly believes in the wrong cult or believes the wrong way (like protestantic instead of catholic) (or maybe the other way around) you end up nowhere.

As pets brains are probably to simple to be able to believe in a god pets will most likely not go to heaven.

It completely depends on what you believe in - nobody knows for sure what happens after death, and if there is a heaven or not. But if there is a heaven and it is your want to have your pet with you, I don't see why not.

I think the Sacred Ibis goes to heaven when it dies.

The Thorny Devil, however, heads in the other direction.

Flying donkey can go to heaven