You and Tweek were friends since you were both children (they are currently 18.and you are both imps, I mean you two live in hell)

You had adopted an infernal pet, they were adorable and they had given it to you (You choose the name). Tweek, upon finding out that you had adopted a hellish pet, immediately became attached to the pet

One day, you were sitting on a bench with Tweek next to you, petting your Pet but when you looked towards him after a few seconds, he saw you with an annoyed look, full of animal fur since your pet was starting to lose his fur so that more new fur grows