Ubuntu, one of the most popular Linux distributions, was released for the first time in October 2004. The first public release, 4.10, was codenamed Warty Warthog. The second and third launches were named Hoary Hedgehog (5.04) and Breezy Badger (5.10). The combination of an adjective and an animal name has been maintained ever since.
With version 6.06 of Dapper Drake, they decided to start alphabetically sorting the code names so that users could easily determine the version based on the nickname assigned to the version.
From Dapper Drake, the Ubuntu Linux version code names were alphabetic. 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 returned to "A" with Artful Aardvark (5:10 pm) and left from there. You can consult the complete Ubuntu version history here.
Image reproduced with the kind permission of Canonical Ltd.