There are those who believe in the reality of creatures that most of us think of as mythical. Who is to say that there is not a real creature behind each of these animals that is considered mythical? Perhaps there is a real animal behind each of them albeit slightly different than some descriptions. As you look at the different dragon pictures that are available think of the similarities to known species in today's world. I also find it interesting to read about the idea that dragons have the ability to breath fire. I think it likely that the dragons of today are actually a combination of many real creatures of the past.
As for the idea that dragons breathed fire I think there are some legitimate things to look at with this idea that may account for stories that eventually lead to dragons pictures that show fire breathing dragons. The ideas that come to mind lead to two or three different possible outcomes. It could be that the dragons spoken of as breathing fire had venom that they sprayed like the spitting cobra. This venom when it comes in contact with the skin and eyes could have given an intense burning sensation. Another possibility is of a similar idea with venom or poison being sent froth from the mouth and causing damage.
The third idea is similar with an excretion that is actually flammable and burns when it comes into contact with flame.If dragons are a conglomerate of actual creatures then it is likely that they have been invented from serpents or reptiles such as the large lizards we have discovered in the world, smaller lizards and perhaps the cockatrice which seems to be either a real creature that had the ability to spit paralyzing venom that also had a negative effect on plants. You can kill any vegetation with a simple solution of soap and vinegar with the soap offering adherence and the vinegar drying out the plant and preventing photosynthesis. It is entirely likely that a species of serpent or lizard lived with such venom.