Once upon a time, when The Ultimate Fighter was new and cool and fighters with no grappling skill could earn a living if they hit hard, there was Melvin Guillard.

Guillard had virtually no ground game, but he could punch a dude in the stomach and score a knockout. So we loved him.

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But eventually everyone ages out of the Octagon, and it’s been years since we’ve seen the dude in action. Oh sure, he’s been eking out a few dollars here and there by fighting on lesser promotions (and usually losing). That’s about it, though.

Oh wait, did I mention that he’s been smashing dudes – and a girl, apparently – in bar fights?

Take it away, TMZ Sports!

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 … knocking the guy out cold before he hit the floor.

The victim was clearly unconscious and people ran to tend to him — while a female patron tried to attack Guillard in retaliation. Guillard hit her too.

TMZ Sports spoke with a Denver Police Dept. spokesperson who told us several witnesses were able to identify the attacker as Guillard … and now a warrant has been issued for his arrest.