Was never a huge fan of Underfell!Sans ’s sharp teeth but I dunno.  The story behind the way I draw it is because it hints he lost that one tooth somehow. Maybe just replaced it with one hold sharp one, a fight perhaps?

On the other hand its the most recognizable thing about that version of him so….idk if I should change it back.

Also instead of a collar I like the bandanna idea.  Of of the rules I have for myself with Sans is he is never allowed not to smile. So another workaround the sharp teeth, maybe when he didn’t feel like talking he did this:

Also my interpretation of him he’s more the type of person that vents frustration outward, more of a brawler so unlike the normal version, he could probably tank stuff pretty well.