How Rainbow got her cutie mark. ( Hehe)
Her and her mother went to an art store where they also had other ponies art work on display for the first time.
Rainbow became so excited that she started opening the paint and decided to "help" the other ponies artwork.
Actually ruining them.

And yes, a little change on her design. In an anatomy topic, unicorns actually have this long tail the fur on the end unlike horses or pegasi.