A newborn Pegasi is extremely small and light. It's wings are shorter than it's legs and are completely useless. They're covered in fur and few plumes, so they're very soft and warm at contact. The foal can't move them until it's two months old; at that age they're still unusable but the foal gets to "know" them and understand how they "work". Pegasi don't have a particular age when they start to fly, it's usually around 4 years after their birth, but every foal is unique and only the strongest and most skilled learn how to rise from the ground when they're 3 years old. When they're 2 years old, most foals start to flap their wings so they can improve their speed when running. Parents teach their foals how to fly by picking them by the tail and throwing them in the air. The little foal starts to move its wings, but at the first attempts it fails and it drops on a soft surface, as planned by its parents. And so, the foal slowly starts to fly, and after the first time it doesn't drop, its parents stop this "exercise" and let the foal learn by itself what to do. Pegasi can have 2 or 3 types of feathers. Most of them have got 3 types: these Pegasi are great at diving and have a strong endurance, but the ponies which have only 2 types are faster and agiler. Feathers are delicate and proof at the same time. They can survive a rock fall, but if dyed or neglected, they can cause troubles to their "owner" and can make flight really difficult and painful. Because of this, they need a constant and frequent care and a lot of exercise every day.
Pegasi are much lighter and, usually, smaller than Unicorns or Earth Ponies. Their bones aren't hollow, unlike birds, but they are thin enough so that the strong wings can lift the pony from the ground. Pegasi have an incredible sight and hearing. They could hear a whisper in the middle of a storm. It's quite rare for a female Pegasi to have more than 2 children. It's ever more unusual that the mare gives birth to a couple of twins, since pregnancy is extremely painful for Pegasi. The winged fetus needs a lot more energy to grow than a non-winged one. This happens with Unicorns, as well.
Role: Racer, Wonderbolt and Element of Loyaly.
Anatomy: Pierced tongue. Her small shape and agile wings allow her intense speeds. Which of course gives her the rare magical ability of creation Sonic Rainboom. This concusive sound is known to break glass or unsturdy houses she may be to close to. She's always aware of her surroundings and is extremely careful where she does it and in what situations.
Rainbow Dash is a normal sized Pegasi, and is about a head or two smaller than her friend Fluttershy, of course she finds no shame in this and constantly tries to build confidence in her friend. Scars she received from dangerous races and Dragons.
Job: She's known as Wonderbolt. She often training young foals in flying and helping them keep watch in their own way. She has a very positive view to children and this doesn’t keep her from occasionally being childish. Thankfully her friends are able to set her straight without any hard feelings.
She is loyal to her morals, if her friends are blatantly in the wrong way, she's able to see it and will boldly speak her mind on the matter. Though because of her hot head and sometimes short sighted demeanor she can sometimes get swallowed in conflict before realizing what is actually wrong, but she's quick to solve any problem. In this she will, sometimes reluctantly, put others needs before her own. What many may call brave and heroic as well, she regularly brushes off as a normal occurrence, unless someone feeds into her ego.
Newborn
Little Rainbow Dash was born as a premature foal, but she didn't suffer nor she had any problems at birth. She laughed instead of crying unlike the other foals who cried or whined. She was very quiet one.
Rainbow Dash was very disappointed that she couldn't use her wings, when she first discovered what those soft things actually did. Her dad often took her on his back and slowly flew around the house, letting her experience the excitement of flight.
Filly
Rainbow Dash was one of the foals that learn very fast how to fly. She first flew it when she was about 3 years old, pretty early compared to the average. When she was 8, she did the mythic Sonic Rainboom, becoming the idol of all her classmates. She was quite conceited and cocky, at those times, but she never mistreated her best friend, Fluttershy. On the contrary, they were often together and Rainbow always stood for her.
Rainbow Dash's mother was the most proud of her after the Sonic Rainboom. Since that day, they spent more time together and raced often, to the filly great's amusement.
Teenager
Now Rainbow Dash was the most known daredevil in all of Cloudsdale. Everypony talked about her, at every corner you could hear somepony say "That's Rainbow Dash!" as she walked by. She grew more and more superb, until she lost the fight that, perhaps, influenced her behaviour most. She was protecting her best friend from a large group of bullies, when she heard Fluttershy behind her sobbing. She turned her head trying to comfort her, but she did a terrible mistake. The leader of the colts hit her, and that was the start of a fight that lasted for the rest of the afternoon. Unfortunately they were defeated, they came back home with wounds but Rainbow Dash got more. Rainbow Dash was more dispirited than ever, because she could not protect her best friend. Fluttershy healed her wounds and curled up next to her for the whole night, crying in silence as Rainbow Dash stared at something far away through the window. Since that huge defeat, Rainbow Dash didn't dare vaunting in front of the other bullies again, but she later took her revenge against each one of them.
Adult
The daredevil, cocky and bold Rainbow Dash we know nowadays. She loves adventures and danger in general, she's always ready to stand up for a friend and she's probably the most sympathetic of the Mane 6. Under that rough shell she hides a sensible heart, tormented by the guilt she still feels for having hurt her parents in the past. She become a Wonderbolt member and after a few months she earned the title of Team Captain.