Ghost pet

Pets, like Sims, leave ghosts after they die.

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.

Ghost dogs will dig ghostly holes.

The Sims 3: Pets
right|300px Ghost Pets can only die of old age and can be brought back to life just like all other ghosts.

Ghost Dogs

 * Can dig holes
 * Can still build relationships with sims
 * Sims with the Dog Person Trait may get the Moodlet to see their ghost Dogs

Ghost Cats

 * Can still hunt
 * Still can build relationships with Sims
 * Can interact with other animals
 * Sims with the Cat Person Trait will also may get the Moodlet to see their ghost Cats

Ghost Horses

 * Can be ridden
 * Still has saddle when they die
 * Sims with Equestrian traits will want get a Moodlet to see their Horse's Ghost