I saw this picture on the front page of Reddit and i was unable to find sources describing the legend and tradition surrounding the babugeri costumes. Any information would be appreciated. Thank you ahead of time!
I am almost certain this is part of the widespread wildman tradition: see this Nat. Geo. article on the topic, and check out the photo gallery for numerous examples. I can't speak specifically about this Bulgarian tradition, but it appears to be related.
What you might find helpful is to look for "kukeri" rather than "babugeri". Kukeri is the more common word used for the practitioners of this custom throughout Bulgaria (usually translated as mummers in English).
