Uropygids are Thelyphonida, commonly called long-tailed whip scorpions or vinegaroons. Mastigoproctus is a particularly large genus of uropygid, and the giant version has a body about 12 feet long. Like other giant uropygids, it is a nocturnal scorpion-like arachnid found in damp and dark places such as jungles, forests and the underdark. An uropygid's tail and front legs are long and whip-like, it uses all three like antennae to feel about itself. Mastigoproctus have strong scorpion-like claws augmented with spikes for piercing prey.

Uropygids are named "vinegaroons" from the vinegar stink of a defensive liquid they squirt at enemies. The uropygid can spray this fluid in any direction it can bend its abdomen's tip, including in front of itself. A giant mastigoproctid is unaffected by this fluid and is highly resistant to all other forms of acid and poison.

Giant uropygids lair in burrows dug in earth or soft rock. Females seal themselves inside a hollow while nursing a clutch of 5d6+10 eggs, then carry their newborn hatchlings about on their backs.

Drow elves often keep giant mastigoproctids as guardians and hunting beasts. The dark elves have bred and magically tinkered with these "pets" for millenia and may be responsible for their resistance to venom and corrosives.

(Uropygids first appeared as Giant Pedipalps (Uropygi) in Q1 Queen of the Demonweb Pits (1980) by Gary Gygax and David C. Sutherland III, the original author was Sutherland)