Something about the ring and note makes you believe that what it's saying is the truth. You're not sure why. Maybe it is magical, after all.
So you pick out a costume. The first costume you see is a horse costume. It's made to be worn by two people, one the front and one the back end. But anyway, you strip--probably unnecessary, but you don't want to test the ring by wearing your regular clothes *and* the costume. Then you put on half of the costume. Suddenly you feel the ring get warm and a strange sensation fill your body, as if you're being pulled apart.
It occurs to you that you're not quite sure how this ring works with a costume. A costume is something worn by one person to make them look like another. So do you turn into the person the costume is supposed to look like, or the person the costume is made for?
Then you realize that there's someone inside the costume with you. And it's still a costume--you certainly haven't turned into a horse. You remove the ring and then take off the costume completely, both from yourself and from the other person. And in front of you is standing someone who looks exactly like yourself, naked just like you.
He says "Wait... you're me!"
"The ring must have--" you say.
Your lookalike continues the sentence. He must have been thinking the same thing. "--turned me into the person the costume is made for, not the one the costume is supposed to look like. I'm two people now..."
You both go down to your room to get some clothes, and then....