Birdhemoth

Birdhemoths, also referred to as world-birds or, derisively, as Time Roosters or poulets temporels, (PROSE: ) were gigantic, intelligent birds capable of travelling between universes. (PROSE:, , )

Nature
The Bookwyrm described the Birdhemoths in their prime as "titanic, peacock-like and radiant" — "creatures the size of a small palace, with wings like rainbows, wings that could pierce through the fabric of reality itself". They were long-lived beings: Rich was already alive over 250 years before meeting Coloth. (PROSE: ) As evidenced by Rich, Birdhemoths were sapient and capable of speech, (PROSE: ) though they also had "celestial trills" which, after being stabled for decades in the Plume Coteries' Library, "atrophied into pitiful squawks". Also as a result of being kept outside their natural element, most Birdhemoths "lost their spirit, their lustre", becoming "paunchy, nearly flightless" so that they "began to look rather less like peacocks and phoenixes, and rather more like common barnyard chickens". (PROSE: )

Steeds of the Plume Coteries
At some point before the Cosmic War, one of the many posthuman coteries "found and tamed the last of the Birdhemoths", becoming known as the Plume Coteries. (PROSE: ) Indeed, Roger had heard from her family that before they changed their name to Bookkeepers, (PROSE: ) the Plume Coteries (PROSE:, et al.) travelled on the backs of majestic multicoloured birds. (PROSE: )

Thanks to these majestic steeds, the Plume Coteries explored the variety of pocket universes, bubble universes and other such not-quite-universes surrounding their native reality. They eventually found the infinite Library. By the time they "realised quite what they'd found", the War had begun and they had grown frustraded with the lack of respect they got from other posthuman coteries. Deciding that they needed a new gimmick, they changed their name to "the Bookkeepers" and settled in the Library permanently, stabling their Birdhemoths. (PROSE: )

In the Library
Over the following centuries, many of the Birdhemoths devolved to much less elegant and powerful creatures, although a few could still fly between universes. Realising they would soon be out of transports, they used some of their last fliers to travel back into the main universe and buy a fleet of Void Ships from the Space Lords, the Shapeships. (PROSE: Indeed, the only Bookkeeper Roger encountered in her lifetime travelled in "a boring metal sphere" instead of the birds she'd heard about. (PROSE: )

After being resurrected in the Plume Coteries' Library, Coloth was originally meant to quickly meet and befriend a Birdhemoth called Rich, who would help him escape from the Library with Maritsa and Callum while he was still a wanted fugitive. However, after arriving in an earlier era of the Library, Auteur decided to alter their timelines to make for a more satisfying narrative, and killed Rich 250 years early, leaving Coloth, Maritsa and Callum to spend months as fugitives in the Library with only ghost-memories of their time with Rich. Eventually, they met up with Auteur, who revealed what he had done but suggested that it wouldn't "stick" and they'd eventually be reunited with Rich. (PROSE: )

"Rushed job, honestly. Some of it might stick, some of it might not… I expect you‘ll find your poulet temporel sooner or later. C'est la vie."

- Auteur (PROSE: The Cactus and the Corpse)

Indeed, an account of an older Coloth showed him travelling with Rich, who was large enough to carry an entire city block's worth of people on his back. While they were travelling outside their native universe and passing through the 10,000 Dawns, Rich's wingbeat caused a "rogue wave of dimensional pressure" which led to a Galaxy Accident befalling the Sun of the beings of light's Dawn. Realising the subsequent plague of Invisible Infernos was somehow their fault, the two stayed in the Dawns, helping Dawn to deal with this crisis as well as others, including the destruction of Dawn 10000. (PROSE: )