Going by this thread, the anonymous user-ID's may be what's left in the posting history, when a user requests that that their account be removed. The guests may be something similar, but with some forum configs, may relate to accounts that existed prior to some point in time, or a period when posting may have been possible without having a full login account.


It might also be possible that a new user could choose a username including the words guest/anonymous (though, I'd suspect that those are likely reserved words that the software would not normally let you use).