Slitherhawks are dragons mentioned twice in How to Speak Dragonese.
There is no physical description, but evidently they are edible. They are probably on the small size, as they are mentioned as being Hunting Dragons.
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 novella 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|