Harvey Bullock from the Batman comics is a Cowboy Cop at times. He often seems to be butting heads with Commissioner Jim Gordon over some aspect of police procedure. He could verge on being a Dirty Cop. He used to hate Batman in particular, but has since developed a sense of respect for him.
Depending on the Writer, Bullock goes back and forth on this. In some works, he's portrayed as being representative of the (often sizable) part of the GCPD that dislikes Batman because they think Batman tramples all over their authority. In others, he's almost as much of a maverick and outcast as Batman himself, and proud of the fact that he "steps on a lot of toes" and "doesn't play politics". And at least one work (Denny O'Neil's Knightfall novelization) states that he dislikes Batman because he thinks that Batman doesn't go far enough in hunting down criminals.
Deputy (later Sheriff) 'Shotgun' Steve Smith also fills the bills. Based on 'Popeye' Doyle from The French Connection, he was a classic cowboy cop who toted a Sawed-Off Shotgun as his personal sidearm.
Bookhunter: Detective Bay would get chewed out by Da Chief in any rational universe. But he doesn't. Ever. Apparently everyone in the Library Police has cowboy tendencies.
John Colby from Chew is gleefully willing to violate every bit of police protocol that gets in his way.
Colby: What can I say, Chu? I'm the unhinged, break-any-rule, loose-cannon cop. You're the by-the-book square that never met a departmental regulation that you didn't love. That's why we work so well together.
He might've backslid recently, as the newest spin-off book, Emerald Warriors, stars Guy as he cowboy cops around the galaxy, punching anyone who tries to tell him about any regulations he's breaking. There might be a reason for the sudden increase in violence though, as the same series shows he's mentally recovering from being possessed by a red ring.
John Hartigan from Sin City both invokes and averts this trope. It is invoked in the sense that he apparently skirts the rules here and there and his actions at the beginning of That Yellow Bastard would be considered police brutality. It's averted since he's the cleanest cop in the city to the point where people treat him as if he were a boyscout.
Superman villain Preus is a very dark take on the trope. As a member of Kandor's Citizen Patrol Corps, Preus frequently ignored procedure and acted on his own in order to secure criminals. Since he was an Absolute Xenophobe who essentially used the rules as an excuse to purify the city this was a bad thing, and even the rest of the CPC thought he was crazy.