An ensouled bear head is an item which can be dropped by bears. It is used to gain Prayer experience by using the level 21 Magic spell Reanimate Bear from the Arceuus spellbook. When a player reanimates an ensouled bear head, a reanimated bear will appear and grant 480 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