Well, as much as I don't believe "real dragons" ever existed, I always find it entertaining to come up with hypothesis on mythical phenomena. Fire breathing could rather easily be explained by the dragon creating a flammable substance out of oil within its body and collecting it in a sac, which then could be spat out along with a spark creating mechanism (grinding of teeth, etc..)Ice breathing would really have to depend on what you mean by "ice breathing". If you mean if the dragon will spit out chunks of ice as its attack, then I'd say he'd just get killed and/or made fun of by other dragons. If you mean it spews out a liquid that would freeze things on contact, then it becomes more realistic (for the lack of a better word). It could be in liquid form, whereby nitrogen from the atmosphere is collected, compressed, and chilled via refrigerator-like mechanism, and stored in a well insulated sac. It could also be an extremely frigid air (also via refrigerator-like mechanism) that is blown out that can chill/freeze things instantly. This of course is based on non-magical dragons. Since dragons might as well have been magical if they existed, then my first two paragraphs are thrown out the window and replaced by a single word "magic".
