So, you are trying to spawn a monster when a player has an item in their inventory? Is this item usable or is it like a bear skin?
You can make a usable item to create a monster, but then I've never tried to give a player an item when using an item, I'm sure that is possible, and thus, you can have the player use an item which summons a creature and gives that item back to the player.
You could get a quest that will trigger when a player reaches a certain resource level and thus, the item could give the player that resource when equipped. The player would then lose the resource when it's de-equipped (I think). Thus, every time the player is wearing the item the quest triggers, and ever time the player is not wearing the ring nothing happens.
Not exactly sure what the end goal is so, I can only tell you this much about it.