I noticed there are questions tagged untagged (actually, they are 8).

Why are there questions tagged with it?
I know that in past it happened when a question was migrated from another site that was not using any of the tag used from the receiving site, but that is not surely the case for some of those questions (for example, https://esperanto.stackexchange.com/posts/1660/revisions).

What made that tag appear in the site? What tags should be used for those questions that would replace the temporary untagged?