Edit: You can find most of these images by using koakuma + dual_persona and koakuma + multiple_persona, but that also turns up many false positives like Koakuma with multiple Flandres.
This is the best place to finally address this in the forum, but someone misunderstood the meaning of モブ河童 when copying the tag from pixiv.
モブ mob; minor character (in computer games, from mobile object) does not refer to the "group of people" kind of "mob". See also the pixpedia entries for モブ河童/mob kappa and モブキャラ/mob chara モブキャラ is the pixiv equivalent of the extra tag we created when K-On! aired. Coincidentally, they also use a K-On example image. If you look through he モブ河童 tag" on pixiv, you'll notice that just like danbooru with extra, they use the tag only for the characters who appeared in canon but are not named.
Someone has started adding tags for the individual kappa such as the dark_haired_kappa. I'm okay with this, since they got quite a bit of attention in the manga recently, although I think when they have individual tags, a group tag isn't needed for tagging that one character. (edit: For example, I think this image shouldn't need 2 character tags: post #1221258) I think we should make a distinction between unnamed characters from official works and original characters made by fan artists.
Dei_Shirou is pretty much the only artist I see drawing kappa and "koakuma" other than the ones from the official media.
Well, actually there are a whole bunch of artists who drew multiple Koakuma. I do agree _mob is not the best naming convention, I only used that because of kappa_mob.
I should add that there is a younger short-haired fan version of Koakuma whom they call Kokoa. Most koakuma ~clone ~dual_persona ~multiple_persona featuer her. Then there's the "dual koa" and "quad koa" joke.