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.