Morpheus' Arms is an item that allows a horse to be aged without aging points, but once this item is applied to a horse it can no longer find aging points. Additionally, if the horse is immortal then it will lose the effect of the Morpheus' Arms at the age of 30. Mortal horses will keep the benefits of the item until death.
Morpheus' Arms can also be found in the Nyx Pack. 
This item costs 3 passes in the Black Market.