When a Sim who has pets dies in The Sims 4, and they were the only one living in the household, the game pops up a comforting message about the pets being taken in by a loving family.
That message sounds nice, but behind the scenes, something very different happens.
Pets cannot stay in a household on their own. They need a teen or adult Sim around to look after them. Once the last Sim who can care for them is gone, the game quietly removes the pets from the save. They disappear completely.
You will not see them walking around as strays, and they do not show up for adoption later.
You never have to worry about abandoned pets wandering the world. At the same time, the truth feels a bit harsh. The pets do not move on to another household. They simply stop existing in that save file.
There is no in-game option to bring them back once this happens. The only way to see them again is to go back to an earlier save from before their owner died.
If you want to recover them, load an old save where both the Sim and the pets are still alive. Open Create a Sim, choose to edit the household, and save that household to your library.
After that, close the old save and return to your current one. From Create a Sim, add the saved household from your library to your game.
The pets will be there again, but their old relationships will be gone. They start fresh, using the default new pet relationship.
It is not perfect, but it is the closest thing the game offers if you want to keep those pets around.
Happy Simming!