Well, Dan goes unmentioned in Revelation, and I think this verse is a relation of the reason as to why it is not.Judges 18:30
And the children of Dan set up the graven image: and Jonathan, the son of Gershom, the son of Manasseh, he and his sons were priests to the tribe of Dan until the day of the captivity of the land.
Read the entire chapter for context, but a serpent is never really a positive thing in the Bible when you are called one. It's one of the titles given to Satan(Rev. 12:9), for example.I don't think this verse is one of the easier ones to understand at first, but if you look a little deeper, the answer is there.