What is "gulltoppr"

In Norse mythology, Gulltoppr ( Old Norse "golden mane") is one of the horses of the gods. Gulltoppr is mentioned in a list of horses in the Poetic Edda poem Grímnismál and in Nafnaþulur section of the Prose Edda. According to Prose Edda book Gylfaginning, he is the horse of Heimdallr. Rudolf Simek theorizes that Snorri assigned a horse to Heimdallr in an attempt to systematize the mythology.