Monster Tail

Monster Tails is an item found in

Uses
Monster Tails are the primary ingredient for the Blue Potion in A Link Between Worlds. Link can bring the Witch ten of these monster tails and she will brew him up a serving of blue potion, assuming Link has the rupees required as well.

Location
Monster tails can be found across both Hyrule and Lorule. Monster tails are not actually an item, but a spoil. They can be found in chests. However, the primary method of acquiring monster guts is by receiving them as a drop from various enemies. Nearly every enemy has a chance of dropping monster tails, along with Monster Horns and Monster Guts.