Slitherhawks are dragons mentioned twice in How to Speak Dragonese. There is no physical description, but evidently they are edible.
Hiccup first glimpses a couple while being taken to Fort Sinister as a prisoner by the Romans. He notices that the closer they got, the less animal life was around.
- "The reason for this was clear when they spotted two dead Slitherhawks all tangled up in a gigantic net, hanging from a cliff face."
- ―How to Speak Dragonese
Later, the Fat Consul is served up a variety of dishes, one of which includes Slitherhawk.
Slitherhawks are also mentioned in the short story How to Train Your Viking, showing that they can be trained as Hunting Dragons.
- "There were a few shiny, slippery Slitherhawks with watchful eyes and short tempers"
- ―How to Train Your Viking
- How to Speak Dragonese (Book 3), Cressida Cowell. 2005
- How to Train Your Viking, Cressida Cowell. 2006