Other.

The cats do not wear collars since they are inside 100% of the time.
Out of the dogs, Frisk is the only one to not wear a collar. Ever since he was young he has never wanted to wander outside of the yard. Even in public, he doesn't stray more than a few feet from me. Of course, though, he is always harnessed when outside of the yard.
Skylar & Ethan both wear collars 24/7, though.