Explain xkcd: It's 'cause you're dumb.
Ponytail is a stick figure character in xkcd. She is distinguished from other characters by her white ponytail hair.
Her name may be Joanna, as suggested in 322: Pix Plz.
A girl named Sarah with a ponytail appears in 84: National Language. However, this comic seems to be about the real-life friend of Randall.
Ponytail was mainly a filler character, showing up when two females are needed or when a large group of people are present. But she has become more and more used, also without for instance Megan in the comic. In many comics where she has a specific role, she tends to have more educated roles such as in 451: Impostor as an engineer. She seems to be typically less nerdy and more energetic than Megan (e.g. 588: Pep Rally).
 See also