There seems to be a bug with the Ring of Divine Might and Horn of Battle.
When a unit is spawned (summoned or gained through grunt deaths, the activatable ability is in effect on them for it's duration (10 seconds)
IE, if you have the Horn of Battle, when a spirit is gained, yeti summoned and so forth, they have +200 health per second for 10 seconds. You just need to have picked the favor item for this to occur.