Prusa / Prusy / Prusai / Preussen / Prūsa / Prøjsen / Preußen / Пру́ссия / Прyссия
In the 13th century, the Polish prince of Mazovia, Konrad, invited the Teutonic Knights to help him subdue and Christianized the Baltic Prusian tribes which were raiding his domain.
The Teutonic Knights did a job by exterminating (and partially absorbing) all those pagan tribes northeast of Mazovia.
After the Battle of Grunwald in 1410, most of these lands went under Polish control, the rest became an embryo of the later, powerful and militaristic state of Prussia, which soon
took over Polish possessions in the area.
The state of Prussia is no more, but nowadays some people are finding their ancestry directly related to the ancient Baltic Prusian tribes.
The language was meticulously reconstructed, flags adopted, and even the "national" anthem (Prusas Grima) was composed and written.
Their aim is to revive the neo- Prusian Nation.
These people reside mostly in Poland, Lithuania, Germany and the US, but also in Belarus, Latvia and Russia.
They are represented, among others, by the Prus Society and Pruthenia Scientific Society in Poland, Romuva in Lithuania, Krivia and Gega movements in Latvia
and Belarus and the Russian Academy of Science programs.Note that there are different version of this flag which you can find in these
Version 1 [link]
Version 2 [link]
Version 3 [link]
Version 4 [link]
Ps: I know perhaps this version of the flag may be a bit off