The Dragon Guardians of Tomorrow fly extremely fast, so fast that they really can't be seen.
|“||That sand... that sand that earlier has seemed alive, now gave birth to creatures of indescribable horror. Who knows what they were: dragons or monsters, or something worse. They were so huge and so swift and so deadly it was impossible to see them.||”|
|— How to Betray a Dragon's Hero|
They are mentioned as having eyes, with a gaze so bright, you cannot look upon them. In another passage, they are described as eyes flicking about like a cat's.
Though the Dragon Guardians lived under the sand on the Island of Tomorrow, their natural habitat seems to be the upper atmosphere or "the edge of space". The shooting stars we occasionally see in the skies are described as actually being the "dragon Guardians, burning at the edges as they fly."
Death By Airy Oblivion
They are dragons which alongside the human Guardians, have been cursed to be bound to the sands of the Island of Tomorrow for a hundred years or until a worthy king is found. They prevent anyone from landing on Tomorrow and punishing them by taking them up to "Death by Airy Oblivion". Death by Airy Oblivion involves grabbing the offending person and shooting up into the upper atmosphere where the victim presumably suffocates and explodes or disintegrates, raining down as dust or purple rain.
We see Death by Airy Oblivion occur almost three times. The first was given to UG the Uglithug who attempted to land on Tomorrow with fake King's Lost Things. Hiccup was also grabbed for not having all the King's Things, but was later dropped by the Guardian Dragons, as he still had one lost thing, and the Dragons recognized his claim for the kingship. Finally, the punishment was meted out to Alvin the Treacherous, who broke the laws of single combat.
Dragon Guardians also have X-ray vision, along with the aforementioned bright gaze.
Additionally, the Guardians produce as high pitched screeching that grates on every nerve. They are able to communicate ultra-sonically, outside the range of human hearing. However, the Human Guardians are able to listen.
As quick as the Guardian Dragons are, they travel so fast that it is difficult for them to stop suddenly, or turn around.
|“||The dragon Guardians had rocketed up into the airy atmosphere, and t would take them at least five minutes to turn around and dive, like unimaginably enormous peregrine falcons, back under the sands of Tomorrow.||”|
|— How to Fight a Dragon's Fury|
In the final Book of the series, the Guardians are seen again, and nearly kill Hiccup, before inexplicably stopping.