8 inches of snow is nothing to a horse - that's not even belly high on a mini. Unless your horse absolutely needs to be someplace warmer, or there is no shelter or wind-break otherwise, just leave them be. If given the choice they will usually stand outside and turn their butts to the wind!
The only time I would truly worry is during icy conditions like freezing rain, or blizzard conditions - and I'm talking BLIZZARD. Like 2-3 feet of snow at once. Honestly during winter and cold, colic is a worse enemy than slipping and falling. Make sure they are drinking enough water and getting enough hay, and they should be dandy.