Wikia

"Erumpent horns, tails, and the Exploding Fluid are all used in potions, though classified Class B Tradeable Materials."

—Erumpent tail's use in potions^[src]

Erumpent tails are long and rope-like. They are originally the tail of an Erumpent. They are classified as a Class B Tradeable Material. The use of Erumpent tail in potions remains unknown though it has been mentioned to be an potion ingredient.