The direct descendent of Odin's eight legged horse, Sleipnir is chosen by Sigurd after seeking the advice of Odin.
In the Volsunga Saga, there are many stories involving this horse. After Sigurd takes the treasure of Fafnir, he places it on the back of the horse and rides away.
Also Brynhild promises to marry whoever can ride a horse through a ring of fire. Gunnar tries to ride his horse, Goti who refues, so he tries Grani. However, he is unable to do this, Sigurd sucessfully rides through the fire, thus winning the contents. This tale is told here.
Information came from the Volsunga Saga and The Saga of the Volsungs, a Penguin Classic.