The Gemara in Megila (12b) says that when Achashveirosh called Vashti to show her off to his drunken buddies she refused even though her own perverse nature would typically make her a more than willing participant to this lewdness. The reason for her refusal says the gemara, is because Malach Gavriel came and made her a tail.
The Likutei Basar Likutei brings from the Orach Yisharim that the symbolism behind this tail was that the Navi Yeshaya says (14) V'Hichrati L'Bavel Shem, Shi'er, Nin, VaNeched"; Hashem will cut off from Bavel every last symbol of remembrance. The word Neched says the gemara (Megila 10b) refers to Vashti. She was the last remaining kin of her grandfather Nevuchadnetzar's house. Therefore Gavriel made her a tail, a "Zanav". Just like the tail which she grew is the end of the animal's body, so too Vashti was the tail end of this wicked family that was finally cut off.