I think the original was told by /u/clanspanker in this comment:

I was at a truck stop one time and this 18 wheeler pulls up at the first stall to fill up with diesel. This butthole with a big mix breed dog was sitting out front with me on the bench. The dog sees a small monkey in the cab of the truck and goes nuts. Barking and jumping at the door so that the driver can't really get out.

The driver rolls down his window and asks butthole to call off his dog. Butthole laughs and says, "Why you afraid my dog gonna eat that little monkey? HAHA!" The guys replies, "If you dont call off your dog I am going to turn my monkey loose on your mutt." Further guffawing from butthole man daring him to do it. So the truck driver puts a little bat in the monkeys hand (you know the kind you used to could buy at Six Flags) who has already started poking his head out of the crack in the window glaring murderously at the dog.

The driver says, "Ok, you asked for it." and finishes rolling down the window. The monkey leaps out onto the dog gripping into his neck fur with his little hand like back feet and is immediately riding on the dog's head. He begins to literally pummel the shit out of that dog with his little bat. The dog takes off like greased lightning running as fast as he can yelping bloody murder with the monkey mercilessly smacking him again and again as they ride off into the sunset. Butthole man's jaw is on the floor and begins to threaten to sue the driver. Myself and another bystander tell him to shut his lousy trap and he storms off cussing like a sailor. Couple minutes later the monkey returns still carrying his bat and deftly climbs back into the window watching his owner to see if he approved. We all approved I assure you.