Because she's a PONY, Susan. Same reason she's got the tail, which I notice you do not question for an instant, which I suppose is fair enough with Grace.
What I'm wondering about is the gloves. It can't be as simple as I thought they would look good with the rest of the outfit, can it? SURELY NOT, and one day, I will figure out just what I'm hiding.
I actually expected more people to know who Jay was without reminder, but good lord, it was MAY when she was in the story comics! It's August now! Time is... What even IS time?
Anyway, fun fact, I wasn't 100% sure how long it would take Susan to get to this point, so part of the reason Susan got a hair cut was so I could be sure this would happen after certain story comics.
Then I got distracted by weird stealth nonsense and the concept of combining games, so I apparently need not have worried?