Ghost pet

Pets, like Sims, become ghosts after they die.

The Sims: Unleashed
In The Sims: Unleashed, dogs and cats, like Sims, do not age and have indefinite lifespans. However, like Sims, they can still die. In Unleashed, unlike later games, pets can starve and die in fires. When dogs or cats die, their gravestones will be different from those of human Sims, and their ghosts will appear occasionally. As with the ghosts of human Sims in the original game, ghost pets in Unleashed will be green regardless of their cause of death.

The Sims 2: Pets
In The Sims 2: Pets, dogs and cats can only die of old age, and their ghosts are white. More often than not, a Sim may spot a beloved pet floating around the house at odd hours, though the Prima Guide for The Sims 2: Pets states that ghost pets will only interact with other ghosts. If University is installed, a pet can be brought back to life with the Resurrect-O-Nomitron. If a ghost dog's former owner is also a ghost, the dog will play fetch with the Sim. Ghost pets also play with each other if they had a good relationship when they were both alive.

Ghost dogs will dig ghostly holes.

The Sims 3: Pets
In The Sims 3: Pets, pets can only die of old age. Their ghosts can be either red if they have less then 10,000 Lifetime Happiness or blue if they have more then 10,000 Lifetime Happiness. A ghost pet can be be resurrected at the Science Facility in the town, but it is possible to restore a ghost pet to their original form with Ambrosia, since pets are able to eat Ambrosia but you need a sim to feed the ambrosia to the pet.

Just like ghost Sims, ghost pets can build relationships with living Sims and are able to interact with other animals. Ghost dogs can dig holes, ghost cats can hunt, and ghost horses can be ridden. An interesting thing about ghost horses is that they still have saddles on when they die.

Ghost puppies, kittens, and foals
Just like Sims can have ghost children, pets can have ghost puppies, kittens, or foals. In order to do this, it is required to have a male ghost pet and a playable female pet if the player wishes the ghost puppies/kittens/foals to be playable. When born, they will have their own headstone in their inventory, and their family tree portrait is grayed out as if they were dead. They do not disappear at the end of the night and they can age. They are technically equal to a normal pet, but they have a ghostly appearance and they can produce ghostly offspring.

Videos
Video:The_Sims_3_Pets_Ghost_Horse|A ghost horse haunting.