To upgrade your pets in the game and obtain a higher degree of rarity, it is necessary to follow specific rules and steps. Below is a guide on how to fuse pets:

1. First and foremost, it's essential to remember that to fuse pets, they must be of the same rarity level.
2. In addition, they should have the same number of stars.

These requirements apply regardless of the rarity tier you are aiming for.

 
For example, to get a pet of:

• Common rarity with two stars
• Rare rarity with one star
• Rare rarity with two stars
• Epic rarity with one star
• Epic rarity with two stars

It's crucial that the pets you want to fuse have the same rarity and the same number of stars.

However, when you're aiming to raise the rarity to 'Legendary' or 'Exotic', there are additional rules:

To get a legendary rarity pet with one star:
1. The pets must be of the same rarity, in this case, it should be two pets with epic rarity.
2. They also need to have the same number of stars. Hence, you must have two epic pets with two stars.
3. The pets you want to fuse should also be the same type, e.g., a Night kitty can only be fused with another.

   

To get a legendary pet with two stars, you need:
1. Two pets of the same rarity (two legendary pets).
2. The pets must share the same number of stars (two legendary pets with one star each).
3. The fusion must occur between the same types of pets.

Lastly, to obtain an exotic pet with one star:
1. You require two pets of the same rarity (two legendary pets).
2. They must have an equal number of stars (both legendary pets with two stars).
3. Lastly, it's mandatory for these pets to be of the same type.

By adhering to these guidelines, you can successfully upgrade and maximize the potential of your pet collection within the game.