It isn’t a scar, but rather a strap to attach her vest onto her barrel. She has another strap further back, which is far more visible.

There’s no other shot with either of these two marks on her so it’s probably an animation error. Even the other picture of her here with color has nothing. The ear part could be an earring though since she could have added it at some point.. And if it was there while grayed out we’d still be able to see a hint of it and not be just invisible. Lightish blue-gray against a darker redish Gray should be obvious to spot

@LightningBolt  

It probably is an error, but at the same time, I really want to believe she actually has a scar there. Mostly because, when pondering what could have caused it, I recalled that there is a perfect implement to have created a wound that approximate size and in that location. Something that she could have carried in a previous profession.  
 
And I mean, thanks to Shining’s buddy Spearhead, we know they do occasionally retire to do other things.  
 
I kind of like the idea of Kerfuffle as having been a guard at some point. Especially with the accent. It just makes it super adorable.
 
“Sorry, this here area’s off limits dontcha know.”

Yeah I doubt its a animation error  
it doesn’t look nothing like an earring.

@i-shooped-a-pwny  
suck my ah shit here we go again

Maybe asking the staff would be the best idea…

Errors.

Maybe they’re connected to the reason behind her prosthetic hoof…

Looks like a little heart-shaped ear stud but it’s not visible on any view but when looking head-on.

Got to be a secret past life.

She has ticks…

secret tattoo