It's awesome that you feel that way guys! I'm sure you would take care of her very well! (except for Derpy Paws, but I still respect his choice http://mlpforums.com/public/style_emoticons/default/dry.png )
If this were to happen to me, I would just die right there looking at Dashie. I would feel shocked, questioning my reality seeing her physical body there, making me go crazy! I would faint over the impossible events happening to me, but when I would wake up, I reboot my mind to the responsiblilities ahead of me.
I would pick her up, and sob for hours, holding her tight. NO WAY would I even think about leaving her there!
I now plan what to do when I take her into my house unnoticed. When I get there, I take her to my room downstairs (It's about 60 degrees down there, but I have really warm blankets). I would try to pray about it to god (Questions like: What to do? How is this possible? Is this a dream or hallusination? Am I going crazy?).
When I finally get my perception on 'reality' changed, I would first show her to my brother...
I would call him down and say "it's an emergency!", then tell him that he will go crazy if I showed him. After I do my best to warn him, I pick up Rainbow from the blankets and show him...
Asuming he would be speachless for hours, I would make sure he didn't reveal her to anyone else. I would tell him what happened, and how I was planning on taking care of her.
Keeping her secret from my family (My parents now get suspicious after seeing me and my brother going down and up from there all the time.), I would finally have her revealed to my parents...
Of course they can't believe it until they calm down, and we would most likely leave the church because something like this wouldn't ever happen... Once my parents chill out, I have them read "My Little Dashie" to find out what happened to the guy in the story.
We keep Dashie secret from the world and we take care of her here for 2 months here. We then move to a place far out of Utah, and in an open space (Like in the story). Me and my younger brothers drop out of school, and I finally show them Rainbow Dash. I would make them believe that if they told anyone about her, then she would die.
Now as a family regaining our lost sanity, we take care of Rainbow Dash expecting Celestia to come less than 15 years from now. I get a lot of bonding time with her and I teach her the things she needs to know (Including the My Little Pony episodes with her in them, so she can adapt to the wierd tragety faster). I would probably have some arguements with her, and sad moments, but I would still love her. I would always love her.
5 years later... (I will start telling this like a story now)
I just turned 19 years old and my family moved back to the city. I bond with her more and more as time goes by, and she still won't stop calling me 'daddy'.
I think of her as my daughter now- no, I KNEW she was my daughter now!
My parents and my brother paid for my taxes and other stuff (Me not having a job and all). I did not care about anything except for Dashie. She was my life now...
I eventually showed her the MLP episode about her doing a sonic rainboom (I didn't show her before because I wouldn't want her to hurt herself trying). She begged me to take her outside so she could try it.
She was already an excelent flyer, but I always told her not to go to fast. She obayed me because I told her that she would catch on fire if she tried. She got mad at me for lying though, but she knew that I had to. After the air was cleared with her, I told her to fly as fast as she could...
BOOM!
I knew she could do it (Well she did it in the fanfic!), but I was still prouder than ever! Luckly, no civilization was close enough for her to break anything with the shockwave... Well, except for the windows of my house... I still didn't care about that.
10 more years have passed, and I was not accepting what was coming...
I read "My Little Dashie" to... well... MY LITTLE DASHIE! It was almost like a religous belief to her because it explained the same events that where happening, and it had a revelation to what was going to happen...
Well to end the story, the same thing that happens to the original character, happens to me.
In denile, I still begged Celestia to find a way for me to communicate with Dash. She still denies my request and still decides to take Dashie.
All of my family were there to say bye to her, but I didn't want to say goodbye.
(I don't expect it, but...) She started wrighting a note to me like in the story... It was a bit different than what I expected, but I still cried just as hard.
After the last hug, and with tears both of our eyes, I told her how much I loved her.
With a tear going down her cheek, she said: "I love you Daddy... I will miss you-" She started crying harder when she reilized that Twilight was going go erase her memory. "You will always be my little Dashie. I love you".
She left with Celestia and the other ponies, looking back at me one last time. I still had my memory, so I wasn't going to stop crying.
I now finished my life with her. It was like someone took- NO THEY DID take my only child! My Little Dashie was gone... She was what gave me great expiriences, but I have to move on now.
Now I have a job, a better house, a wife, but I refuse to have any kids. No one could replace my little Dashie. She was always my little Dashie...
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I'm so sorry everypony! I was aiming for a short, sweet, realistic scenario, but I drifted off and started to make it a short FANFICTION!
I started making myself cry while I was writing, and I got carried away...
I tried making it as short as possible. If I make a fanfic, I'll work harder on it, and I wont be posting it as a reply to my own topic!
You don't have to read it (If you haven't already). I didn't work really hard on this, I just started to say what was coming out of my mind.
WELL, THAT'S MY STORY!!!