https://en.m.wikipedia.org/wiki/Origin_of_Hangul
"However the character 古 gǔ also functions as a phonetic component of 蒙古 Měnggǔ "Mongol". Indeed, records from Sejong's day played with this ambiguity, joking that "no one is older (more 古 gǔ) than the 蒙古 Měng-gǔ". From palace records that 古篆字 gǔ zhuānzì was a veiled reference to the 蒙古篆字 měnggǔ zhuānzì "Mongol Seal Script", that is, a formal variant of the Mongol ʼPhags-pa alphabet of the Yuan dynasty (1271-1368) that had been modified to look like the Chinese seal script, and which had been an official script of the empire."
Might be true, might be not. Still interesting to see. And the Mongols themselves mostly stopped using their script to write in Cyrillic and Hanzi (?) now so.
We have another example of such great men creating a new script by himself after exposure to another script. Cherokee by Sequayah.
First is, of course, tengwar, but no real spoken language uses it.
Second if hangul, which is great overall and has an easy structure [2], with just a few historic warts.
Third, there's Cree syllabary. Just look at the logic: [3].
[1]: https://lotr.fandom.com/wiki/Tengwar#Spelling_and_pronunciat... [2]: https://sites.google.com/site/hangulanationallanguage/photo-... [3]: https://fineartamerica.com/featured/plains-cree-syllabics-tr...
Hangul is far less capricious, though, a marvel of careful design.
Do you have a source on that? As a Bulgarian (where Cyrillic comes from) i had never heard anything of the like, and a short Google, in Bulgarian or Russian, found nothing.
https://ru.wikipedia.org/wiki/%D0%A2_(%D0%BA%D0%B8%D1%80%D0%...
https://ru.wikipedia.org/wiki/%D0%AE%D1%81_%D0%BC%D0%B0%D0%B...
From there the transformation into "Я" is obvious. And it happened gradually over time long before Peter; he just happened to look at various different shapes already in use during his time, picked the one that looked the simplest and the most appealing to him, and (sometimes) did further minor tweaks to simplify.
It has absolutely nothing to do with the shape of the letter "R".