A Rat's tail is a quest item used in the Witch's Potion quest. It can be obtained by killing any rat in RuneScape after the player has begun Witch's Potion. The item has a 100% chance to drop once you begin the quest.
Brian's Archery Supplies, a store that sells Ranged supplies, has a couple of rats which are very convenient for the player, since the store is located near Hetty the Witch's house.
After completion of the quest, the Rat's tail is unobtainable unless you kill a rat, complete the quest and then pick it up. If the player shows a Rat's Tail to the Wise Old Man after completing the miniquest, he will tell the player to get rid of it. (Before completing he says "You have brought me a tail of a rat!" and "Yes, I think you should keep it for now.")
Dropping monsters
Monster Combat level Quantity Rarity Rat 1 1 Always