An ensouled unicorn head is an item which can be dropped by unicorns as well as black unicorns. It is used to gain Prayer experience by using the level 22 Magic spell Reanimate Unicorn from the Arceuus spellbook. When a player reanimates an ensouled unicorn head, a reanimated unicorn will appear and grant 494 Prayer experience after being killed.
The head can be reanimated anywhere across Gielinor only if the spell is cast on the head while it is still on the ground. Once the head has been picked up, it must be taken to the Dark Altar in Arceuus to be reanimated.
For an exhaustive list of all known sources for this item, see here