I'm thinking of uploading a pretty old piece of r34 fan-art I made back in July that feature characters from one of my favorite artists (who also allowed me to make r34 fan-art of his characters) but I'm not sure about the tagging rules regarding fan-art of someone else's characters. Should I tag both the artist behind the art (me) and the owner of the featured character(s) as well?

Also, is it allowed to create tags for original species and character names of someone's original webcomic series or should I only use tags that already exist (tagging unknown_species instead of tidri, which is an original species).

Only the actual artist gets an artist tag on the image, so it'd be only yours on the image. Feel free to make a new tag for the species and character name, if there isn't one already.

I see, I guess I'll just credit the character's owner in both the drawing itself and the post's description.

what they said. Personally I use the generic tags (avian/scalie/mammal/alien/monster...) rather than specific unless I'm uploading three or more pics, but that's just me, since I don't like single-use tags for things other than artist, copyright or nonfan characters/species.