Interestingly there's a pretty good argument that Cyril and Methodius didn't create Cyrillic at all but instead were the creators of Glagolitic, a now-more-or-less-extinct script that was developed in what's now (roughly) the Czech Republic and was used in Croatia until the 20th Century: https://en.wikipedia.org/wiki/Glagolitic_script

Exactly how Cyril got credit for the script he didn't invent is unclear.