Eight-Legged Nadders are mentioned in the reference book, The Incomplete Book of Dragons.
Eight-Legged Nadders, as their name implies, have eight legs, and are sinuous dragons classified as Cave Dragons. Based on their image, they come in blue and purple colors. As with most Book dragon species, Eight-Legged Nadders have a spaded tail.
To note, the picture of two Eight-Legged Nadders shows one that appears to have poison glands on the neck. The author frequently draws poison glands or sacs in such a way. However, nothing is mentioned about these Nadders being poisonous.
Though these Nadders appear no where else in the Book series, they have a "Disobedience" score of "4". This indicates that they are indeed trainable by Vikings. Because they are on the smaller side, they may have been used as Hunting Dragons (as opposed to Riding Dragons).
It is implied that Eight-Legged Nadders may be pack animals, or at least form loose groups. They sleep together closely, and as a result, can actually get tangled up. The resulting 'knot' of Nadders can take days to sort themselves out.
Eight-Legged Nadders are described in this Reference Book.