Pets like Toddlers and children can not live by themselves so the game culls them when the last Teenager or adult in a household dies. There is no way to get them back as they no longer exists. You only option would be to load a savegame prior to this happening.
Sadly, I think @jpkarlsen is right.
If you have an old save where the pets still exist, you could save them to your library and get them back that way. You would load the old save, edit in CAS, save the household to your library, then close and open the new save. Then edit your university household in CAS (fulleditmode?) and choose add a household from your library and import the pets that way. They'll lose their relationship, unfortunately, and will have the friendly default new pet relationship.
