Not to be confused with the Rainbow pet.
Rainbow pets are a new mechanism added to Pet Simulator as of Update 5. When hatching an egg, there is a very low chance (0.5%) of the pet turning into a Rainbow pet. Rainbow pets can also be acquired by combining 7 Golden Pets of the same type.
In contrast to golden pets, any pet can turn rainbow, even if it is a reskin. However, rainbow pets cannot be made golden. Rainbow pets can be turned into Dark Matter Pets by combining 5 Rainbow Pets of the same type.