Ex-UFC fighter Melvin Guillard -- who fought the likes of Nate Diaz and Justin Gaethje -- brutally attacked and KO'd a man in a Colorado bar ... and now cops are on the hunt.
TMZ Sports has obtained surveillance footage which shows the 35-year-old getting into it with several people at Bar Standard in Denver around 2 AM on Monday morning.
After some pushing and shoving, you can see Guillard -- with his trademark bleach blonde hair -- cold-cock a bar patron in the face .