A bit of wild mass guessing on my part.
So, in the first My Little Pony special, there was a mysterious white pony. She had the hair colors of a Rainbow Pony, but none of the Rainbow Ponies released by Hasbro had the same cutie mark. As it turns out, the only appearance she had outside of the special, was a porcelain figure who went by the name 'First Born'.
My theory is that, some time after the Tirak incident, First Born left Dream Valley and ended up in G3 Ponyville. Some time later, she gave birth to Rainbow Dash. This idea came to me when I realized that both First Born and G3 Rainbow Dash not only had the same hair color, but also the same cutie mark.
As to what caught filly Rainbow Dash's attention, I'll leave up to your imagination.SVG Download
My Little Pony (c) Hasbro
Friendship is Magic (c) Lauren Faust