It is a poison gland taken from the tail of a radscorpion, giant radscorpion, or albino radscorpion. A normal radscorpion has a single poison gland while the giant and albino varieties have four.
Uses
Poison glands are a component used in the construction of the dart gun (Fallout 3).
At weight 1 and a value of 30 caps, (value of 25 in Fallout: New Vegas) radscorpion poison glands have a very high weight-to-value ratio, higher than any other miscellaneous item short of certain unique or quest-related items (and weightless items). Because poison glands are effectively limitless in supply they provide a reliable way of making money throughout the game.
Radscorpions can be found all over the Capital and Mojave Wastelands (though they are less common in urban areas).
Notes
The poison glands in Fallout 3 differ from those in Fallout: New Vegas by being more shriveled and desiccated. The Fallout: New Vegas poison glands are more plump and smooth.