Since fire breathing is an iconic part of dragons and fire magic common in fantasy, it has been a important feature to include. A simple fire mechanic is going to be in the next patch and here some more info about it.
Most things that feel to be able to burn will do so. This includes wooden walls, trees, most animals and guests. Fire deals damage to animals and guests, so burning guests will try to run away from the zoo. Buildings and other static things will break if they burn for too long, this allows for fiery animals to escape from unprotected wooden enclosures.
To combat new fire mechanics workers now try to prioritize important jobs such as extinguishing fires and run while doing them. There is also new crystal of fire that prevents fires in its range so you can still hold dragon in wooden enclosure.
With fire comes 2 new spells to play around with it. More important spell is the rain spell, which can be used to extinguish fires in large area (it might have some other logical effects too). Fire spell is meant to be a more of a fun spell and allows hurling fire into your zoo. So if you want to cause some chaos you are allowed to.