The name Sky Dragons seems to refer both to a species of dragons, as well as to a class of dragons elucidated in The Incomplete Book of Dragons. The classification contains dragons such as the Windwalker, Silver Phantom, and the Bullrougher.
As a species, they are referred to in both A Hero's Guide to Deadly Dragons and How to Ride a Dragon's Storm. The Murderous Tribe sacrifices intruders to these dragons, which live in their tribal territory, the Murderous Mountains. Also to note, the Murderous Tribe generally isn't very descriptive with dragon names. Examples are the Stealth Dragon, Sniffer Dragon, and Storm Dragon.
This is seen again in How to Ride a Dragon's Storm:
- "Perhaps it was their uncomfortable habit of sacrificing unwelcome intruders to the Sky Dragons at the summit of Mount Murderous that kept people at bay."
- ―How to Ride a Dragon's Storm
Madguts the Murderous saught to sacrifice Stoick the Vast and Big-Boobied Bertha to the Sky Dragons as a condition of winning the Inter Tribal Swimming Race, because Stoick's ancestor Grimbeard the Ghastly sacrificed his ancestor, Murderous the Magnificent to the Sky Dragons as a reward for winning the first Friendly Intertribal Simming Race (not very friendly).
Above their heads, the Sky Dragons were already amassing in the hundreds, with shrieking, hungry vulture cries.
- A Hero's Guide to Deadly Dragons (Book 6), Cressida Cowell. 2007.
- How to Ride a Dragon's Storm (Book 7), Cressida Cowell. 2008.
- The Incomplete Book of Dragons, Cressida Cowell. 2014.