Dear Changeling known as 'Stupid',
First, and above all, I apologize on behalf of Anon who gave you your current name - since he probably won't. If you would like to select a name more fitting for yourself, I highly encourage it as I would hate for you to be demeaned simply by introducing yourself, or being addressed. If you would like a name given to you, just let me know. If you already have one, I of course suggest you use it unless you would prefer not to.
Now, on to the actual content of your letter.
A long time ago, Equestria was founded by ponies who were starving, whose every day was a struggle, whose lack of trust and what was seen as their base natures drove them against one another. It was founded when ponies put aside what made them different as things that should keep them apart, and instead celebrated them for what made them special. Whether we have feathers or horns or even fangs, Equestria is, and has always been, about coming together, new chances, and above all, hope.
As one of Equestria's Princesses, I welcome you, if this is truly what you wish. I welcome you to make friends, to find love not just to eat but to share and to experience. I welcome you to find what truly makes you special, what makes you you, and find the light inside of you and make it shine.
I welcome any pony, changeling, or other being that is willing to try for these things, to live in harmony and work together to make a life worth living. With your consent, I would like to study the Elements effects on you with the aid of researchers of the crown, so that others may benefit, if they so desire.
I wish you well,
Princess Celestia
To my most faithful student, Twilight Sparkle,
I have received the missive from our changeling guest, and would like to amend my previous letter thusly: Twilight, you are not allowed to run experiments on him without informed consent. You are not allowed to act in a manner which is threatening to his wellbeing - physical or psychological - except to subdue if an escape is attempted. You are also not allowed to permit others to break any of these rules for you because I have not forbidden them from doing so.
I was just thanked for such luxuries as 'not starving' and the chance to live in peace, Twilight. One of our core tenets is Generosity, and that means when we have food, we share it. When we are warm and someone else is cold, we let them warm themselves by our fire. When someone is hurting, we help.
Make me proud, Twilight,
Princess Celestia