Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
ponytail [ˈpəunɪteɪl] n (hairstyle) хвост* хво́стик
to have one's hair in a ponytail носи́ть*(impf) хво́стик
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.