There are several like "drawings & paintings" and "digital art" that come from an early version of my DeviantArt parser, which parsed DA categories as unnamespaced tags. I don't think the parser tries to grab those any more, I think because their html changed or something.
I also notice, now you mention it, a bunch of "watercolor (medium)" tags. I don't know where those came from, but putting the namespace in parentheses after the tag is usually a booru thing, so I expect they came from one of those.
The 'hydrus' way to do this is probably "medium:graphite", although there might be something better.
If this is something you care about, please feel free to add whatever makes sense for you to my public tag repo, and if you like the idea of using a 'medium' namespace, please add "ink (medium)->medium:ink" sibling pairs if you know how to do that. There might be some appropriate parent relationships as well.
medium:lurid goop