Hi Hi Puffy AmiYumi was a Cartoon Network series that ran from 2004 to 2006 and was about the adventures of two rockstars named Ami and Yumi who have misadventures around the world. The series was highly popular for Cartoon Network at the time and even has a fanbase on the Internet (although very small nowadays). What most fans don't know, is that the series had a pilot created for Cartoon Network in 2003 that never aired.
After Sam Register (the creator) pitched the idea of Puffy AmiYumi having their own cartoon series on Cartoon Network, the animation studio Renegade Animation created a test pilot, in the hopes of swaying Cartoon Network to green-light the show's production.
Not a lot more is known about the original pilot, except for a few pictures and a short video of it, giving a quite different impression of the series than what would later be aired; for example, Ami and Yumi looked very different than what they did in the series. Ami in some shots had ponytails rather than buns, and Yumi had a more Grunge look to her and even having a fang stick out of her mouth; the animation was also very jerky at times.
The pilot was sent to Cartoon Network and they accepted it and it was to air on late 2003, but for unknown reasons, the pilot was reworked and later premiered on November 19, 2004. The series premiere was successful and was even at the time one of the highest rated shows to premiere on Cartoon Network.
A preview of this pilot was shown on Cartoon Network DVDs and VHS tapes in the early 2000's. It is unknown if people are in possession of copies of the pilot episode. It is possible that the creator or people involved in the making of the show may have a copy but until further footage is available, it is a mystery what the series what would have been like in its early stages.
UPDATE FEB 15 / MAR 28: A promo for the pilot has been around a month ago, but hasn't been posted in the article. You can find it here.