These mutated Great American cockroaches, colloquially called "radroaches" or simply "giant cockroaches", are enlarged versions of the pre-War species that have increased in size, presumably due to atomic fallout caused by the Great War.
Not nearly as common as in the Capital Wasteland, radroaches mostly inhabit abandoned homes and buildings (such as the Basincreek Building) to scavenge on food that was left behind by whoever previously lived there. Some can also be found in the toxic dumps and on/around the roads in between Goodsprings and Primm, as well as being able to survive in the hostile and toxic environment of the Sierra Madre and the Cloud.
Radroaches are not normally aggressive if the player character keeps his distance. They are however, very territorial and will attack if the player seems to threaten them, either by having a weapon drawn, invading their habitat, battling another enemy nearby (which is limited to less than a foot around the creature), or attacking them directly. Generally, they will spread their wings and make distinct clicking sounds in warning before attacking the player.
The irradiated radroach appears the same as a "normal" radroach but with the radioactive aura of a feral ghoul reaver and the ability to heal itself with a radiation blast similar to a glowing one. It does not appear in Fallout: New Vegas without the use of console commands. It does appear
in miscellaneous folders as a remesh, however.