The cooldown period for the Guardian of Nature appears to be about two turns, and on the third it will attack. It first appeared on the 114 turn and vanished on the 124, in my game. I'm unsure if it has a limited amount of health or not.
Upon arriving it gives the message: "The power of your Ice, Magma, and Thunder dragons summons a Guardian of Nature." Upon leaving it leaves the message: "Guardian of Nature leaves to maintain the balance."
Hopefully this information is useful for the Wiki, have a good one.