pixiv's Oldest Drawing is a tag describing the oldest drawing in pixiv, and submissions that started everything since the establishment of the service.
The oldest drawing is determined by the illustration ID, and no matter which site you go to, single digit illustrations are submitted as a test before it went live. While the first submission ID is obviously 1, it is no longer available in this site.
Hence, the first illustration was submitted by Bacotu (馬骨).
Interestingly, as Bacotu is the first user of pixiv (as he is the one that created pixiv), he is the only user with the ID which is lower (10) than the official pixiv staff ID (which is 11), making him the first user in pixiv.
The Legendary Illustration
Everything started with a kettle...
The second oldest
And the third oldest