Radscorpion

Radscorpions are an extremely large version of the Emperor scorpion, which was prolific in pet stores at the time of the bombing, mutated by radiation. Contrary to some expectations, their poison grew more potent, not diluted. They are nocturnal and sensitive to light, however on the East Coast this is not the case, as giant and regular size radscorpions have been spotted in the daytime. Samples of poison from their tails can be used to make antidotes. Apart from their size, they look pretty much like normal scorpions, with shell color ranging from red to black.

Giant Radscorpions
Simply a much larger Radscorpion with a highly poisonous stinger that will deal 40 damage every 5 seconds. 4 Radscorpion Poison Gland are found upon death, as opposed to the usual one. They have no known weak spots, so be prepared for a fight. (I have noticed that all radscorpions seem to take more damage when you target their front claws, although it may be a fluke)

Glowing radscorpions
Similar to normal radscorpions, glowing radscorpions are a bit more ferocious and much more radioactive. They're so radioactive that they glow (naturally). That means that just getting near these creatures will cause you some damage, and their claws will cause damage by radiation as well as their tail stinger.

Spitting radscorpions
Spitting Radscorpions are a step sideways on the evolutionary ladder from normal radscorpions. They have developed the ability to spit radioactive venom at distant targets. As a result, their claws are severely under-developed, used only for protecting the spitting radscorpion's head. Spitting Radscorpions are usually yellow in color with a bright red coloring on their back and legs.

Appearances in Fallout games
Radscorpions can be encountered in all Fallout games.

Related quests

 * Stop the Radscorpions

Bugs

 * When the Animal Friend perk is activated, the Giant Radscorpions are not hostile anymore and will not attack you. Star Paladin Cross kills them, even when they aren't hostile.

I have a single rank of the Animal Friend perk (and I'm running the PS3 version of the game) and have never encountered this bug.