Flag zombie has a flag. A red flag in fact. In theory, this could possibly be a Nazi flag from 1944. This could be true due to the flag being red and the stick being not very long. This could also mean this could have been from a Nazi soldier after invading a country. They could've raised their arm up with the flag up.
They tried to cover it with a brain sticker but I know his trick.

After seeing the small evidence which makes so much sence. I can say that the flag zombie is a Nazi Soldier.
Note: He also is trying to get new flags. But that still won't stop me.

I made a shocking revelation.
During a SMG4 video. SMG3 uses the zombie "laugh" and then mario kicks his ass and specifically says it being a "Nazi Zombie" so clearly it shows that now EVERY ZOMBIE IS A NAZI.
This might be the biggest revelation ever.