Alrighty, I'm basically just doing this to amuse myself while I work on some other projects. This is going to be a list of what I think are the relevent kin for the unsorted demons in the demon list and a little bit of explanation as to why I think so. This will not effect main articles in any way and is just my dumb little fan interpretation. It also won't be done all at once, but what can ya do...
Kin of Amaimon[]
Bariyon - Confirmed Kin
Dekalp - Confirmed Kin
Ent - Given that Dekalps and Greenmen are already known to be Amaimon's kin, it would stand to reason that other plant based demons might fall under his purview as well.
Goblin - Confirmed Kin
Greenman - Confirmed Kin
Hydra - Confirmed Kin
Imp - No specific traits, quite similar to the Goblin.
Komainu - Powerful physical abilities, but no elemental traits.
Malkin - Called a garden spirit, seems earthy in its style.
Naga - Confirmed Kin
Oni - It's name literally includes the kanji for earth and from what I've managed to translate so far, possesses no trait beyond outstanding strength, so it's probably one of the beasts under Amaimon's sway.
Yeti - Confirmed Kin
Kin of Astaroth[]
Coal Tar - Confirmed Kin
Chimera Zombie - Confirmed Kin
Fungus Demon - Confirmed Kin
Ghost Rider - Given their tendency to hop smoke spewing vehicles and corpses, I think they fit with Astaroth's kin.
Ghoul - Confirmed Kin
Impure Baron - Confirmed Kin
Impure King - Confirmed Kin
Impure Princess - Confirmed Kin
Koks - Confirmed Kin
Naberius - Confirmed Kin
Zombie - Confirmed Kin
Kin of Azazel[]
Byakko - Confirmed Kin
Cat Sídhe - Confirmed Kin
Evil Ghost - Confirmed Kin
Fetish - Confirmed Kin
Furfur - Confirmed Kin
General Frost - Confirmed Kin
Ghost - Confirmed Kin
Gufu - A not-entirely-physical being that controls wind, would seem to fit alongside Sylphs and Vayu and the like.
Indra - Confirmed Kin
Jack Frost - Confirmed Kin.
Jack-in-the-Box - Confirmed Kin
Set - Confirmed Kin
Shapeshifter - Confirmed Kin
Sylph - Confirmed Kin
Vayu - Confirmed Kin
Kin of Beelzebub[]
Bael - Confirmed Kin
Belzébuth - Confirmed Kin
Chuchi - Confirmed Kin
Kin of Egyn[]
Divine Hawk - Mostly due to the water manipulation aspect I think this demon may well be associated with Egyn.
Kraken - Confirmed Kin
M.O.L.B. - In an odd case where the demon came before the king, M.O.L.B.s first appeared in the Blue Exorcist movie, while Egyn wouldn't appear until much later in the manga. So I think with him essentially being a child, and these things being giant slimy water balloons, it seems like something he probably tried to make once upon a time.
Naiad - Confirmed Kin
Oceanid - They're basically just larger, more powerful Naiads, so they're more than likely kin of Egyn, the only reason they aren't here is basically out of professionalism since they haven't been said to be his Kin.
Ouroboros - Seems rather stout and reptilian, vaguely reminds me of amphibious type animals. Very loose affiliation here to be frank.
Reaper - Confirmed Kin
Sea God - Confirmed Kin
Sea Monk - Confirmed Kin
Varuna - Varuna has been shown to control water, and as we've seen other vedic deities and such like Vayu and Indra fall under the sway of the Demon Kings, so it seems likely that this is where Varuna falls as well.
Kin of Iblis[]
Amon - Confirmed Kin
Asmodeus - Comfirmed Kin
Cyclops - Considered a god of fire and iron and given its ability to deep fry things at a distance, I think it's quite probable that Cyclops would be under the Kin of Iblis.
Hinoyagihayao-no-kami - Another fire god, since it doesn't seem to possess any special traits like Yamantaka, I'm gonna assume it could fall under Iblis' influence.
Karura - Confirmed Kin
Muspell - Confirmed Kin
Peg Lantern - Confirmed Kin
Coyote - Seems to be fire based and such.
Salamander - Confirmed Kin
Ucchusma - Confirmed Kin
Kin of Lucifer[]
Canaria - I think these are the little baby headed winged friggers, but the picture isn't clear enough to be sure.
Phantasma - Confirmed Kin
Seraphim - Confirmed Kin
Kin of Samael[]
Black Dog - Confirmed Kin
Das Stärkste Gefängnis - Confirmed Kin
Death - Confirmed Kin
Kuchens Kuckucksuhr - Confirmed Kin
Phantom Train - Confirmed Kin
Shadow - Confirmed Kin
Usamaro - Confirmed Kin
Kin of Armumahel[]
Heilong - Confirmed Kin
Mahakala - I mean, it's made of Black Flame, I think that's good odds its related to Armumahel.
Yamantaka - Confirmed Kin
Galatea - I won't lie, this one is more of a pet theory and honestly given Galatea can be made from other demons and such seems like they'd better suit being unaffiliated, but I think the Takara family are Nephilim of Armumahel and as such I think Galatea, as Nemu's primary familiars, fall under his sway.
Kin of Shemihaza[]
Golem - A demon seemingly created through magic that is subservient to a master and is docile when not threatened. While generally this would look like a candidate for unaffiliated, I feel the nature of this particular demon seems like something Shemihaza may have come up with herself as part of the teaching humans to combat demons thing.
Unaffiliated/Unknown Demons[]
Baba Yaga - Given that they are possessed witches, it seems likely that there's no strict affiliation for a Baba Yaga, so I figure here is the most likely place for them.
Dragon - Since at least two dragons seem to exist with unique traits, I think this is more a broad category with individual specimens being affiliated to different kings.
Gehenna Gate - This thing seems to be solely in the control of Satan, so it would probably be considered his kin before any other, but since there's no category for Satan in the Kin system as far as we know, here it goes.
Gremlin - Confirmed Kin
Harpy - Confirmed Kin
Mara Dragon - Since there only seems to be the one of the thing and it's a massive frickin' monster, I'd say fair odds it's an unaffiliated outlyer.
Necrophager - Confirmed Kin
Nine-Tailed Fox Spirit - Confirmed Kin
Three Wise Monkeys - Confirmed Kin
Ukobach - Acts as Mephisto's familiar, but doesn't seem to fit the trend of his own, or any of the Demon Kings he's affiliated with, so I think Ukobach is probably unaffiliated.