Fight Night

 is an unmarked quest in Fallout: New Vegas.

Quick walkthrough
In the middle of the Thorn is a large Arena, and surrounding the sides are cages with Wasteland creatures grown & kept by Red Lucy for Arena battles.

If you like you can set up custom fights by talking to Red Lucy. You get to choose what creature and how many of it will fight in red vs. blue team. Since the creatures are expensive to breed, keep and maintain you will have to pay for the fight. If the red team wins you get your caps back plus a little extra. To initiate the fights simply talk to Red Lucy and select let's talk about The Thorn and ask What's fighting in the next match.

You can also tell Red Lucy that you want to pit against the creatures yourself. You can choose what and how many of it you want to fight, then be teleported to the arena cage. The winnings are not much even if you fight 3 Giant Radscorpions, which scores only 120 caps.