I have no idea what to tag this with beyond the basics. I guess those clothes are traditional Hungarian clothes, but 5 minutes on Wikipedia and two search engines did not shed any light on them, so I don't know.
Added on 2021-03-09: I noticed there's a translation of the above description in DA's comments, posted by DA user EgonDaLatz:
I decided that I'll dress the ponies up in traditional Hungarian clothing. I wanted to draw them like this due to this music theme. I love Hungary.