From Tar Valon Library
Nightlily is a dark mare belonging to a woman called Nemaris. Her husband gives Siuan the horse when she requests the mildest he can find claiming that she is as meek as milk-water. Siuan is such a bad rider however, that she finds Nightlily too frisky, partly due to her own expectations, but mostly due to her attempting to copy things she has seen experienced riders do.
"Nightlily here's as meek as milk water, Aes Sedai. Belongs to my wife and Nemaris is on the delicate side. She don't like a mount what gets frisky"
(Reference: Knife of Dreams, Chapter 1).