There are many pictures of girls who have a snake tail instead of legs. Japanese artists typically call these girls "lamias," but I think in the vocabulary of western fantasy they are usually "nagas."
Both tags exist on Danbooru and they overlap quite a lot.
As for the origins of the words, the Greek "Lamia" was a specific monster, like Medusa, who had a snake's body and the breasts and head of a woman. "Naaga" were a Hindu race of beings associated with snakes and the earth, opponents of "Garuda" associated with birds and the sky. Naaga have wildly differing forms in their depictions; sometimes they're entirely humanoid, other times they are a giant snake with multiple heads, etc.
I would vote for "lamia" as the tag for this, but in any event we should decide on one and get rid of the other.
Updated by Deus