Rainbow Dash is not like other ponies. She possesses magic.
Now, all pony tribes have various levels of magic, but Rainbow's is far stronger than most ponies. Her magic level is right on par with Celestia's and Luna's.
However, it is not a good magic. Rainbow's magic is 100% black magic.
She can only thank King Sombra for teaching her how to use it when she was a little filly.
And now, she must keep this secret away from her friends. She must tell nopony her secret.