Saxony-Weimar-Eisenach – Flag in Lexicon and Shop https://flaggenlexikon.de/fdtsxwe_me.htm
Saxony-Weimar-Eisenach was a grand duchy in the German Empire until 1918. The country used a black, yellow and green striped flag from 1897.
They have their roots most likely in the crest of the Dominion of Landsberg.