"I named it Gyaos, because of the sound he makes!"
Eiichi on the reasoning he named Gamera's new enemies Gyaos. (from Gamera vs. Gyaos, 1967
The Gyaos are a species of vampire bat like kaiju created by Daiei or what is now Kadokawa that first appeared in the 1967 Gamera film Gamera vs. Gyaos. They are the most notable enemies of the titular kaiju Gamera and are his arch-enemies.
Also known as Gaos and Shadow of Evil, the Gyaos resemble a giant pterosaur or bat creature, with a flattened, arrow shaped head, leathery wings with three claws on each, taloned feet and a flat tail. It is reddish brown in both incarnations. The Gyaos in Gamera: Guardian of the Universe had a more streamlined appearance, with larger wings and a longer neck, as well as red eyes after its evolution in Tokyo.
In the Showa series, they have no definitive origin although in Gamera vs. Guiron, it is revealed that there are multiple silver space-faring Gyaos, suggesting that Gyaos may be an extraterrestrial species, or that there are multiple species of Gyaos specific to other planets.
In the Heisei trilogy, Gyaos are the result of genetic engineering by the ancient Atlanteans in order to create a genetically perfect organism, possibly as a weapon. However, the asexually-reproducing Gyaos began breeding out of control and turned on their creators.
The Atlanteans created Gamera as a last-ditch effort to stop the Gyaos, but their civilization was ultimately destroyed by the Gyaos. While Gamera successfully destroyed most of the Gyaos, several clutches of their eggs survived into the present day and were able to hatch due to human activities causing a decrease in the Earth's levels of Mana.
The main antagonist of the final Gamera film in the Heisei trilogy, Irys, is also considered either an alteration or the ultimate form of the Gyaos.
Alongside other Kaiju, the Gyaos do not appear in literature and despite being villains they are primarily the inspiration for the character of Zebiccoon from The Mammal Wars as he shares many similarities to the Gyaos including being half its size, the ability to shoot lasers from his mouth and even the Showa Gyaos roar thanks to them being bats.