From the time before BB and Foxpy ended up with the skeleton brothers. Part 6
“Foxpy tried a full week and more to ‘rescue’ BB without success. He tried everything he could, but Sans never left the bunny alone.
Halfway through the second week, the fox was exhausted and a snowstorm fell on the town. Having spent his magic reserves to the limit and without eating what was necessary to fill them, Foxpy had no chance of seeking refuge. But lucky him, something cool happened.
Papyrus found the orange-fur ball in the middle of the road and always willing to help, he took him home. What a surprise for the two who were waiting for him. FF: Foxpy did a soft purring like sound when Papyrus picked him up, that's what started the whole "it's a cat" thing.“