Ubuntu, one of the more popular distributions of Linux, was first released to the public in October of 2004. The first public version was 4.10 and bore the codename Warty Warthog. The second and third releases bore the names Hoary Hedgehog (5.04) and Breezy Badger (5.10). The combination of an adjective and an animal name has been maintained every since.
With the release of version 6.06, Dapper Drake, they decided to begin alphabetizing the code names so that users could easily determine the version based on the nickname assigned to the release.
From Dapper Drake onward, Ubuntu Linux release code names have been alphabetical. For example, there have been the following names over the years: Feisty Fawn (7.04), Gutsy Gibbon (7.10), Hardy Heron (8.04), and Maverick Meerkat (10.10). When they reached the end of the alphabet with Yakkety Yak (16.10) and Zesty Zapus (17.04), they rolled back over to “A” again with Artful Aardvark (17.10) and have continued from there. You can check out the full Ubuntu version release history here.
Image courtesy of Canonical Ltd.