In the Norse mythology, the day-night horse is actually 2 separate horse; Skinfaxi, meaning "shining mane" and Hrímfaxi "rime mane" (or "frost mane"). Skinfaxi is the horse of Dagr (day) and Hrímfaxi belongs to Nótt (night).
The most obvious ability of the day-night horse is to change it's color according to the time of day, essentially becoming a personal clock. Especially useful to determine the time of day when in the woods (where the sky is blocked by the trees).
The color (as it changes to their night form) also have a reddish hue during the day of the Blood Moon.
A less obvious ability of the day-night horse is that the horse's speed varies according to the time of day. It reaches peak speed at dawn or dusk, and slows down to about average speed around midday or midnight.
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