Magnesium Oxide can be used for many purposes, a calming effect on Horses, for bone formation, metabolism & nerve and muscle function.
As Magnesium is a naturally occurring mineral found in the horse's body, Magnesium oxide is safe for horses to consume. The body doesn't absorb it unless it's needed; any excess Magnesium will be excreted in urine or droppings.
There is evidence to suggest feeding salt with Magnesium increases the animals update of the magnesium. 
Recommended dosage:
10g /day for a 500kg horse in light to moderate work.