Fight Night

 is an unmarked quest in Fallout: New Vegas.

Quick walkthrough
Walk up to Red Lucy and initiate conversation. Ask what's happening in The Thorn. and choose speech option "I am ready to fight in The Thorn."
 * Select which creature you want to fight then how many.
 * Proceed to killing the creature/creatures
 * Return to Red Lucy and receive your caps.

Detailed 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 team you bet on 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 payout are according to what creature that you fight, 3 Deathclaws will give you 540 caps, 3 Giant mantis' will only give you 30 caps. You must complete Lucy's quest to fight stronger creatures, such as Nightstalkers and Deathclaws.

Bugs

 * If you kill any creature with energy or plasma weapons, the ash or goo pile will stay in the fight arena forever.
 * Sometimes a friendly creature will spawn inside your cage area. There is no penalty for killing them, as it counts for the quest.
 * Sometimes you can run through the gate on your side, allowing you to get the drop on your enemies.