Arena

The Arena mutator is a mutator present in every Tournament and Championship game which replaces the game's weapons and ammo with a specific one. The Instagib mutator can be considered part of this, to some extent.

Unreal Tournament
The Arena mutator as per se doesn't exists here. Instead, five different mutators allow the players to play with specific weapons. These are:

There are two extra Arena mutators in the game's code, which aren't available without modifying the file Botpack.int

UC/UT2003/UT2004
This time, Epic decided to get rid of the above, and put all the Arena mutators into a single, configurable mutator.

In order to choose the weapon, you must go to the Mutators tab, and then to the button Configure Mutator. There, you have a droplist where you can choose the weapon.

Unreal Tournament III
This mutator is present again, but under a different name, Weapon Replacement.