So a few things about the Stonka.
First things first remember that article about the water bull that was slayed by an equipped bull.
https://bnr.bg/en/post/100467575/ox-cow-and-bull-in-folklore-imagery-and-tales
Well for my understanding Water Bulls or in Bulgarian Водният Бик/Vodniyat Bik are folkloric creatures found around Bulgarian villages. For what I got the story goes that they are violent bulls that terrorized the villagers until slayed by an equipped four legged animal and also have strong connection with water. The Bulgarian Wikipedia of the Slavic Water Spirit even mentions the bull being a water spirit and does give some location of it’s encounter. The article that was founded was one from Kostenets with one which I found record from 1941 for magazine that can be read below along with other .
https://www.360mag.bg/posts/118838
Here’s one from Tishanovo (Page 85; this one couldn’t detect the language being Bulgarian so I just translate only the bit I need for a cleaner translation at Google translators).
Here’s variety from Tishanovo that’s slightly different from the one above.
https://georgealall.blog.bg/lichni-dnevnici/2012/09/30/iz-spomenite-na-moia-pradiado-pavel-iovchev-kmet-na-s-tishan.1004781
https://www.academia.edu/17300553/Mount_Sacra
http://www.bgnow.eu/news.php?cat=2&cp=4&newsid=36508
Here’s one from Ugyrchin (page 78; had to use image translated it).
http://sfera.logica-bg.com/treasures/XII-05.pdf
Here’s one from Sveshtari (page 15-16; this one also couldn’t detect the language being Bulgarian so I had to us so I had to uses Google translators again; this one had two buffalos rather than a bull named Karaman and Sakar with one of them slaying the water bull and their owner being named Demir Baba or the Iron Father)
https://gutenberg-bg.com/ludogorie.org/wp-content/uploads/2013/10/%D0%91%D0%B0%D0%B1%D0%B0-%D0%96%D0%B5%D0%BB%D0%B5%D0%B7%D0%BD%D0%B8%D1%8F-%D0%B1%D0%B0%D1%89%D0%B0_%D0%91%D0%BE%D1%80%D0%B8%D1%81-%D0%98%D0%BB%D0%B8%D0%B5%D0%B2.pdf
However the one that strikes me the most is this one from Rabisha.
http://www.vidin-online.com/legendi/legenda-za-vodniyat-bik-i-selo-rabisha
The story was said to come from a book from 1969 called “Шепот от вековете. Легенди от Видинския край”. While not identically to the Stonka story it does have more similarity compared to the other sure as The bull being defeated with something made of gold (which is the bulls horns), the water bull able to bring earthquakes and storms, but the most striking comparison is that the bull has one eye on his forehead (and another on the back of his head). I felt this is the closet we reached to the Stonka.
The main problem that none of these stories features a man slaying the bull, so wonder where that version originated from.
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Okay, I finally have to release your great research into the wild.
While you’ve reached mostly the same conclusions and stories about the “water bulls” as @indecisive-grenadier did years ago (though they were never posted publicly as far as I know), I don’t think I heard the Rabisha story with the single eye and the gold weapon. That does sound like the closest we’ve gotten, but I think we are still a bit off.
Take the profile of Stonka from Megami Tensei II no Subete, probably the oldest one available of any MT materials:
ブルガリア、ユゴスラビアの、怪力で知られる不死身の牛。二本の鋭い角で、牧場の牛馬を殺しまくった。馬の皮を着て馬に化けた英雄が、黄金の剣て刺し殺したという。
An invulnerable bull from Bulgaria and Yugoslavia known for supernatural strength. He slaughtered farms’ cattle with his two sharp horns. A hero disguised with a horse’s skin stabbed him with a golden sword.
And SMT4′s profile for fun and completeness (which seems to be functionally identical to the Stonka profile in Kaneko Works I, which itself is probably from that one SMT1 book I don’t have with all the demon art and their profiles):
A one-eyed bull that caused chaos around the Balkans. With a cry like thunder and speed like lightning, it would run through ranches, ramming and killing any cow or horse in its way. Its skin was made of bronze and could deflect arrows with ease. It was also very cautious, never getting close to a human when it spotted one, and could sense traps with its magic. It was finally killed by a man who made a golden sword.
Then, take a look at the profile translations by Miwasatoshi on Stonka’s talk page. Only source three is from a printed publication (and source two is just the SMT4 profile), indeed the World Yokai book from the early 70s. Importantly, the MT2 blurb above and the World Yokai profile are essentially the same, only I suppose MT2 didn’t have the room to mention Stonka’s knack for detecting traps. But MT2 definitely got the information from World Yokai.
But where did World Yokai get its information from? That’s the mystery. But thinking about it, the fact that Rabisha has stuff also in the SMT4 profile that World Yokai/MT2 don’t is probably a good sign you’re on the right track.