The name Sky Dragons seems to refer both to a species of dragons, as well as to a class of dragons elucidated in The Complete 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 How to Ride a Dragon's Storm and A Hero's Guide to Deadly Dragons. 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
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 Complete Book of Dragons: A Guide To Dragon Species, Cressida Cowell. 2014