Ghost pet

Pets, like Sims, leave ghosts after they die.

The Sims: Unleashed
In the original Sims, there was no death of old age (since sims were practically immortal), but sims can die of other, more unnatural deaths. In The Sims: Unleashed, there was also no death of old age for dogs and cats, but they could die. Unlike later games, Pets could die in a fire and of starvation. When a dog or cat dies, their gravestones will be different from those of human Sims, and their ghosts will appear occasionally. As with other types of 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.

Ghost dogs will dig ghostly holes.

The Sims 3: Pets
right|300px In The Sims 3: Pets, pets can only die of old age, cats and dogs ghost color will be red or blue upon death while unicorns and horses are always red. A ghost pets can be be resurrected at the Science Facility in town.

Just like the sim ghost, pet ghost still build relationships with sims and interact with other animals. Ghost dogs still can dig holes, ghost cats still can hunt, and ghost horses still can be ridden. The interesting thing about ghost horses is they still has saddle when they die.

Ghost puppy, kitten, and foal
Just like sim, pet can have ghost puppy, kitten, or foal. It required a male ghost pet and a female pet. A ghost pet is translucent like a ghost, they have their own headstone in their inventory, and when you look at their family tree their 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 basically just like a normal pet, but one that looks like a ghost and can produce ghostly offspring.