Re: Where is my Dog?

@Sirhc2269Aww, I'm so sorry about your furry friend. I love my pixel pets too but unfortunately if the dog was at a solo Sims home and that Sim died... pets can't remain as a solo household and the game automatically deletes them. It's the same with babies and toddlers. You don't get a notification if a character is automatically culled, just if they die.
The game understands rules of ownership by the current household, not the first point the character joined the game unfortunately.

So as I see it your options are if you have an old save and could find the dog sitter's household and then add it to the library/gallery from a previous save, you could then pick it up in your current save but it would be like a new character if I remember correctly. Or you could start playing your game again from that previous save point and add some extra characters to the dog sitter's household to be on the safe side or move him to the family household and turn off death/neighbourhood stories.
This is why I play with death off and Neighbourhood stories off to stop this from happening.
Or you could just create the dog again and tell yourself this never happened. Add it to another friend or family house and build the relationship again, then once your Sim leaves Uni just go to Manage Worlds and split household to move the dog to your Sim's household again. That's what I have done when I've lost Sims before.
I wish we could do that in real life, just hop into CAS and recreate a beloved pet!😞

Edit: you can take pets to university with you if you live in your own housing. Either in one of the Britechester lots or you can change the status of one of the on campus buildings to Residential and live there with pets. Darkwing House is one I use a lot and my most recent Sim lived in Briny Tower which I edited to be a residential apartment, she has pets there.