Cops have arrested ex-UFC fighter Melvin Guillard for that brutal bar attack in Colorado last week -- the SECOND alleged attack he's been accused of carrying out this year.
We broke the story ... Guillard was allegedly captured on video beating the hell out of people at Bar Standard on Sept. 3rd -- knocking at least one man out cold.
Officials issued a warrant for his arrest -- but didn't have to look too hard for the guy ... since he had a court date in CO on Sept.