She should appear if you use the fast travel maps during the day (day/night have different random events). The chance is high enough that it shouldn't take that many attempts, is there any chance you already met her and she is at the barracks?
Viewing post in Caliross, The Shapeshifter's Legacy comments
Oh, that's the problem! The fast travel doesn't refer to the horses, but to the maps around Tarkas. The city has a bunch of maps (they look like signs) to teleport around the various districts/points of interest. Try those, hopefully you should get the event quickly. ^_^
To see how one looks, you can check the square near your house, to the east. One of the maps is located there.