Giant ant queen

Giant Ant Queen are the queen of Giant ants. They are often deeply burrowed in underground caverns and rarely appear on the surface.

Fallout 3
There are only three Giant Ant Queens in Fallout 3. There is one in the Corvega Factory, one in Marigold Metro Station and one in Shalebridge.

Ant Queens are not very fast, though they do seem to notice you from a distance and shoot Ant Spit with surprising accuracy. The queens are generally considered one of the "boss" characters in Fallout 3 as they have extremely high health (about half that of a Super Mutant Behemoth) making them one of the few creatures in the Capital Wasteland that can survive a Nuka Grenade blast.

Marigold ant queen
The Marigold ant queen is the queen of the Fire Ants that have been harassing Grayditch. She was mutated by Doctor Lesko in an attempt to create smaller ants with a chemical mutagen that would be injected into the Queen ant, but instead fire-spitting giant ants resulted.

Related Quest:Those!
During the Those! quest, Doctor Lesko will ask you to kill the Fire Ant Nest Protectors that are guarding the Queen so that he may get to her and fix the faulty mutagen. After doing so, all of the fire ants that were terrorizing Grayditch will start fighting each other, and upon your arrival to the surface; they will all be dead or neutralized, not attacking the player but still showing up red on the radar.

Notes (Marigold ant queen)

 * She is protected by five Fire Ant Nest Protectors.
 * During Those!, Dr. Lesko asks you not to harm the Queen in any way. However, any followers you may have will automatically attack her, resulting in the death of the Queen or your follower. It's best to leave them outside the chamber while you go about your business.
 * It is possible to complete Those! without ever encountering the Queen.
 * You only receive 25 XP for killing her.(No matter what difficulty).
 * If you kill her, you will receive good Karma.
 * In order to save time the Ant Queen Pheromones can be looted from the corpse and given to Angela Staley to seduce Diego in Rivet City.
 * She is apparently not hostile against Dr. Lesko.

Quests

 * A Nice Day for a Right Wedding
 * Those!

Bugs

 * The queen can sometimes be found stuck in the walls near the ceiling, or may go though walls when near nest.
 * When defeating her guards in Marigold Tunnels (Ant Burrow Section), if she catches sight of you, she is known to appear right behind you for odd reasons, even though she clearly would not fit through the tunnel.
 * A glitch may occur where once you kill the Nest Protectors and leave, the Marigold ant queen will glitch/crawl through a wall near the exit and stand in your way, trying to attack you. with some effort you either can jump over her legs to the side and escape (or, of course, kill her)
 * There is a glitch that may occur when you get close to the first defeated nest guardian where the queen will teleport to your location.

Fallout: New Vegas
The  which appears in Fallout: New Vegas can be located in the Ant Mound. It seems to lack much of the durability of it's Fallout 3 counterpart, as a strong weapon and Entomologist can easily one shot it without the need for specialized ammunition.

Van Buren
Giant Albino Ant Queens are found in the Hoover Dam Scum Pits. Unlike the Giant albino ant, their exoskeleton is not very pale, but only their cocoon.

Appearances
Giant Ant Queens appear in Fallout 3 and Fallout: New Vegas, and the Giant Albino Ant Queen was going to be featured in the cancelled Van Buren.