As they were in the early stages of developing Star Wars: The Clone Wars, George Lucas and Dave Filoni were looking for ways to give Anakin Skywalker a padawan. The news was a surprise to even Filoni, but Lucas reasoned that the character was vital in showing how Anakin needed to learn, and failed, to let go of his attachments. The character, initially known as Ashla, became Ahsoka Tano, and the once unpopular padawan is now a fan favorite preparing to wrap up her journey in The Clone Wars. One of the questions Lucas and Filoni sought to answer was why we hadn't seen Ahsoka in the films. As it turns out, we nearly did...
I think it was the name [Ashla] given to one of the [young Jedi Yoda was training] in Attack of the Clones. There was a little Togruta girl. We kicked around the idea that maybe that was Ahsoka but then we thought the age didn’t really work out for it to be the same character. She was too young in the film.
The character in question was one of the younglings training with Yoda when Obi-Wan Kenobi sought answers for his lost planet. She can be seen on the left during this scene:
WHAT THIS MEANS
This is an interesting tid-bit that would've provided a neat connection for Ahsoka to the films but, as Filoni said, it just wouldn't have worked. When we meet her in The Clone Wars, Ahsoka is about 14 years old, and the youngling above is clearly far younger. She also didn't coincide with Ahsoka visually, as Filoni has said that Anakin's padawan was always going to be orange. This could have been one of the greatest unintentional setups of all time, but at the end of the day the retcon wasn't really necessary. For now, Ahsoka Tano still awaits her live-action debut in the Star Wars universe, though if the rumors are correct she shouldn't have to wait much longer. Seeing Ahsoka grow from being Anakin's young (and occasionally irritating) padawan, into the mature leader that captures Darth Maul has been nothing short of incredible, and, if the timeline had added up a bit better, seeing her as a youngling in Attack of the Clones would have made her evolution all the more remarkable.