Deadly Nadders are long and serpentine, and at least come in the colors of green or purple. Their bodies are so long and rope like, that they can get tangled when sleeping together for winter hibernation. In "How to Train Your Dragon", they have yellow cat-like eyes and ears (presumably this is referring to ear pinna (flaps)).
In The Incomplete Book of Dragons, another Dragon called the Eight-Legged Nadder appears. Because the Eight-Legged Nadder looks exactly like a Deadly Nadder, has the same stats and has the name 'Nadder', it is possible that they are the same species and that 'Eight-Legged Nadder' is just another name for 'Deadly Nadder'.