Giant radscorpion

Giant radscorpions are much larger versions of the radscorpion with an even more effective stinger.

Giant radscorpions often travel and fight in pairs. It's not uncommon to see one turn from the player and back off, only to be joined by another and charge in again. In fact, they seem to operate in pairs most often, only preferring to attack alone when their partner is dead.

Fallout 3
Their health is on par with that of a Mister Gutsy robot or super mutant master. Additionally, giant radscorpions lack a "weak point" that can be targeted for extra damage (they have no "head" to headshot), which makes V.A.T.S. combat and firearms somewhat less effective against them. Crippling their legs with mines makes the fight much easier, as their charge becomes much slower. As with radscorpions the dart gun is not very effective against it, slowing it down less than other creatures. They are affected by the Entomologist perk.

Fallout: New Vegas
In Fallout: New Vegas Giant Radscorpions seem to not give chase immediately after sighting you, their "range of attack" is usually about 5 meters from you. It is advisable to avoid these creatures for the first 10 or so levels, as they have a high Damage Threshold, rendering them very hard to kill at early levels (unless you can get your hands on Armor Piercing Ammo).

Loot

 * A maximum of four radscorpion poison glands can be found on a dead giant radscorpion. These have a base value of 30.

Bugs

 * Because of their relatively large area, closeness to the ground, and complexity, giant radscorpions can easily fall foul of one of the many mapping glitches and become stuck in rocks, trees, pillars, or simply in the ground. If you see one thrashing around madly but apparently getting nowhere, this is probably what has happened.
 * If you use the dart gun against 2 giant radscorpions (shooting a dart at one first and a second dart at the other one), the weakest one will disappear from sight while the other is still engaging.
 * A glitch that seems to be especially frequent with Giant radscorpion corpses is that they can become contorted and stretched out, sometimes spinning as well.

Appearances
Giant radscorpions appear in Fallout 3 and Fallout: New Vegas.