Fight Night

Fight Night is an unmarked quest in Fallout: New Vegas.

Quick walkthrough

 * 1) Talk to Red Lucy.
 * 2) Ask what's happening in The Thorn. and choose speech option "I am ready to fight in The Thorn."
 * 3) Select which creature you want to fight then how many.
 * 4) Proceed to killing the creature/creatures
 * 5) 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. The team you bet on is the first one that comes up in dialogue, and it is also always on the left. Red team is on the left, blue on the right.

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, nightstalkers, and such dangerous creatures. 3 deathclaws will give you 500 caps, while 3 giant mantis' will only give you 30 caps.

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.
 * Explosives, thrown or launched run the hazard of injuring the crowd and it may cause them to turn hostile.
 * Sometimes you can run through the gate on your side, allowing you to get the drop on your enemies.
 * Sometimes during a fight you can't enter your cage and get stuck in the metal doorway that opens up leading into the arena. Thus you are stuck in the inner cage area and can't get out unless you load a previous save. So it is highly recommended that you save before starting every fight. Not the case for me. The steel door has some sort of invisible wall in the middle. I just walk in and out to the side (left) of the door, hugging the wall.