Most people believe dinosaurs became extinct before any humans existed, but many cultures have legends of creatures called dragons that closely resemble dinosaurs. This fact indicates that the extinction of dinosaurs must be very recent. (There are some who believe that there are still living dinosaurs. http://www.newanimal.org/dinosaurs.htm ) Some of these dragon legends speak of dragons that breath fire. Is it possible that such a creature actually existed? The best way to answer that question is to look at an animal that we know does exist, the bombardier beetle. Here is what Wikipedia says about it.
Bombardier beetles are ground beetles (Carabidae) in the tribes Brachinini, Paussini, Ozaenini, or Metriini—more than 500 species altogether—which are most notable for the defense mechanism that gives them their name: when disturbed, they eject a hot noxious chemical spray from the tip of their abdomen with a popping sound.https://en.wikipedia.org/wiki/Bombardier_beetle
The spray is produced from a reaction between two chemical compounds, hydroquinone and hydrogen peroxide, which are stored in two reservoirs in the beetle’s abdomen. When the aqueous solution of hydroquinones and hydrogen peroxide reaches the vestibule, catalysts facilitate the decomposition of the hydrogen peroxide and the oxidation of the hydroquinone. Heat from the reaction brings the mixture to near the boiling point of water and produces gas that drives the ejection. The damage caused can be fatal to attacking insects. Some bombardier beetles can direct the spray over a wide range of directions.
Bombardier beetles defend themselves by expelling a hot liquid from their bodies. What if some dinosaurs had this ability but expelled the liquid from its nose or mouth? Because dinosaurs are much larger than beetles the amount of liquid would be much greater and if it landed on something flammable it might even be capable of starting a fire. To an observer it would appear that the dinosaur or dragon was breathing fire. Fire breathing dragons might not be a product of human imagination but something that actually did exist at one time.