Fluffy's backstory, as she tells it, is wacky, so whether or not you think its true or not is up to you.
She says that she grew up in Canterlot, where she claims she was once an Earth pony. Her parents tried in vain to teach their little filly to be a proper mare, but she refused to listen, instead sneaking out at night just to gaze out at the lights, or perform crazy tricks and stunts. She enjoyed circuses, festivals, and fairs, often begging her parents to let her go, which they reluctantly did. Often, Fluffy had to attend fancy parties, in which she hid away from her parents, and constantly visited the buffets.
Fluffy had an odd lack of friends, having only one. This friend encouraged her dream to be an acrobat. She and Fluffy often painted together, another one of her favorite hobbies. Fluffy joked that if she couldn't make it into the circus, she would become a professional painter. She gained her cutie mark when she entered a picture of a blue bird into a contest and it won. Unfortunately, Fluffy's friend had to leave, promising that she would one day come back. This led to Fluffy making tons of imaginary friends.
After leaving home, Fluffy attempted to join the circus. She was an amazing acrobat, and she had graceful skill, while also pulling off crazy stunts. However, the true excitement in her life came when she went to a festival and found an odd booth filled with, as the merchant said, "mystical items." There, Fluffy found a brown notebook with a blue dot gem on it. She wanted to purchase the item, thinking it looked nice, but the merchant refused, saying that "this special notebook is not meant to be used by a silly mare" and that someone else had claimed it. Fluffy, intrigued by this, continued to persist on buying it, but the merchant continued to refuse. Finally, she stole it, wanting to see what was so special about it that she had refused all the money she had offered from her family's fortune.
Fluffy Cloud doodled in it, and it was around this time that odd occurrences began. She doodled a bunch of puppies; the next day a herd of puppies stormed the circus tent. She drew a silly picture of it raining marshmallows; the next day the weather ponies were baffled by the storm of white puffy pastries. Eventually, Fluffy connected the dots and saw that whatever she doodled seemed to occur in the real world. She continued to draw in the notebook, however, because she thought it was fun to mess around with it.
Fluffy left the circus after it was disbanded, and instead went off in search of another job. She earned a job at a clothes shop in Ponyville, and she found a home in Ponyville, and by finding a home, she doodled a real estate pony slapping a 90% off sign on a house. And the next day she found the price for the home dropped.
It was then that Fluffy Cloud doodled more in the notebook, and one time, she drew a portal for laughs, thinking that a portal couldnt possibly form in her home. In a few minutes, a portal opened up in her home, which she was sucked into. There, she doesn't describe what she saw, saying that she only saw "things never seen by pony eyes." She pulled herself out of the portal, and when she came out, she saw she had wings.
After that, more things happened that she refuses to describe, but she says she resigned from her job at the clothes shop, and instead got a job at the Weather Factory, where she mainly works on snowflakes. She also locked up the notebook, and she carries it around with her so she never loses it.
Now, Fluffy is seen as a rather weird but friendly pony, living a seemingly average life. She only tells her story to close friends. It's up to you to chose to believe her story or not. However, any more prying leads to her getting unusually quiet.
