Thanks Onionleaf!
I still don't remember buying this outfit, but I guess I must have! What's strange is that I usually buy every outfit that is released for Dawn, and it's not in her wardrobe. I'm wondering if maybe Dawn purchased it, and it somehow ended up in Sky's wardrobe? I'm guessing Dawn must have let her borrow it, but Sky never got around to returning it
As far as her arrival day, Sky arrived September 19th, 2012, and it says that her Yearling Badge was earned on June 21st, 2013
This must be a bug