If already placed, I'd probably keep the fire phoenix since even at level 4, it gives a 20% increase in attack to all your troops. We don't know if more phoenix artifacts will be available. If they are, it will most probably be in the next spring event. If you haven't placed it, I'd be tempted to place it, but if I didn't, I'd keep it and the artifacts until the next spring event and see if more artifacts will be available or can be crafted during it. If they can, I'd disenchant the one in inventory and craft a new one since it will be for the chapter you are in at that time.
As for the brown bear, we don't know if more will be offered in the fall event when the brown bears were introduced last year. If you have placed the brown bear in your city, it'd probably be best to sell it and use the land for something else. If you haven't placed it, I'd just hang on to it and disenchant it when it's known artifacts will be available for it. You hold on to it in your inventory so it won't keep showing up to be crafted taking up a slot in your magic academy.
As for Sky Essense, it is like the Seeds of the May flower event. They only exist during that event. Next spring Sky Essense will most probably be the item that falls from the sky around your city.