Here is a creature that might qualify as a cryptid, but one for which I think I can work out a plausible explanation without actually venturing into the realm of unknown animals.
In 19th Century England, ironworkers had a strange belief. Unless they let the fires at their factories go out from time to time, they believed a dangerous creature would be generated in the flames.
They do not seem to have harbored any idea of what this creature looked like, but they adhered to the practice. How had such a notion grown up? Had they once forgotten to douse the fire, with the result that something monstrous and horrific had emerged?
Perhaps. But I think I can offer a simpler explanation.
I think this idea owes its existence to the malady known as Obsessional Compulsive Disorder (OCD). Sufferers of this unpleasant syndrome feel that unless they adhere to certain ritualistic behavior, something bad will befall them. They develop habits like having to turn off light switches with their elbows, needing to touch a certain part of the wall before they enter their front door or, at a simpler level, fearing to step on cracks in the sidewalk. We are creatures of habit. If we grow into the habit of doing something and then neglect to do it, we may at firts feel unease and then, if we suffer from OCD, real fear. Fear can then spread to others who do not have OCD.
I can imagine this particular idea of the flame beast began with workers dousing the flames from time to time until this became a habit. From being a habit, it grew up being a compulsion, perhaps among the more nervous workers at first. Then people began to ask themselves why they were always so meticulous about putting out fires. Someone suggested that it must be because, if they didn't, something unkenned would emerge from the fire and wreak ghastly vengeance. The belief, which would have had its origin in one foundry, would have, with the passage of time, spread to others.
Of course, I may be wrong. Perhaps, if they hadn't put out the fires, something unimaginable might have sprung from their flames to devour everything in its path. But I don't think so.