Sketch of Pinkie Pie for 2008 MLP:FiM pitch bible. An early concept that Pinkie would randomly attack ponies with surprise parties. For whatever reason---- birthday, 1 month anniversary of last birthday, congrats on getting your mail, etc....
I think things worked out for the better in the decision of making Pinkie Pie an Earth pony instead of a Pegasus. It makes some of the things she does much funnier since the show is a cartoon. Rule of funny ya know.
I like how they made Pinkie into an Earth Pony and Fluttershy into a Pegasus. It make's more sense to me because how would Fluttershy be able to fly with the birds and reach high up in tree's? And Pinkie would probably be a faster flyer than Rainbow Dash from all the sugar she has. xD
This was Pinkie's original design, she was a Pegasus, but since Rainbow was also planned to be a prankster, they didn't want to have a stereotype that all Pegasi were silly and all Earth Ponies were good with nature, so Pinkie's wings were given to Fluttershy. Pinkie was based on the G1 Pony named Surprise. I'm not sure why they made her into Pinkie... and pink, but more likely or not legal reasons.