Paradoxical as it may seem - existence does not lead to truth, even if truth indicates existence; such relation is non-mutual. Everything thinkable is either existent in the Physical/Scientific World (where things exist for real, independent of experience), or exist as mental constructs (thought, feeling), abstract concepts (logic, law; things are either represented by things and beings the Physical World; a representation that affects reality itself such as a proven scientific theory), and illusions (things and beings that only exist by their name and/or definition, from fantasy creatures to the Flying Spaghettis monster; entities and objects of either fiction or delusion).
The chair I’m sitting on and, for this case, orcs - both of them exist, but not in the same way. While the chair I’m sitting on is real (otherwise I would’ve fallen on the floor), a mythical being exists as a character of a one or more tales, i.e, it is an illusion that exists as such, but can only exist through being represented by something else which exists in the Physical World, i.e, a text on a book we call a “tale”. Without these representations, including our mind as a representative, these mythical creatures wouldn’t exist as illusions, and, thus, wouldn’t exist at all, because illusions require to be represented to exist, with their elementary origin of representation being our minds. If those who invented the illusions of these creatures wouldn't exist, then their own creations wouldn't exist at all (or, at least, be invented by someone else or some other people).
Allow me to represent this logic in a diagram:
Communication (writing, speech, animation, art and so forth)
Collective Consciousness (more than original representative)
As you see, The illusion is the most basic resource that cannot exist on its own, and is represented by all of the rest of the concepts. The illusion is embodied in an idea, the idea is represented by the mind, the form of communication represents the mind and the collective consciousness is the presentation of the various forms of communications, used to deliver illusion-containing ideas, generated by the mind.
This is the evolution of illusion delivered by systems of representations. However, when it reaches the collective consciousness and gets sunken in it for enough time, followed by enough popularity - one or more people begin to wonder if the illusion, the basic unit, is actually a non-illusion, i.e, something that exists in the Physical/Scientific World, instead of what it really was - an illusion; a resident of the Abstract World, or, to be more specifically, the Mental World, because abstract concepts, like logic and laws, can be evident in the Physical/Scientific World, whereas the Mental World can be described as a producer of illusions, which either exist in the private mind or the collective mind - but not beyond it, like abstract concepts from the Abstract World do.
What leads to this lack of distinguishing between these two Worlds is ignorance, of not knowing something for sure. Where knowledge is missing, ignorance is the void; where one is convinced while in ignorance (or without sufficient or evident data) - there is belief or faith. Believing, in contrast to knowing, is always a combination of being convinced (or convincing oneself) while being not entirely sure. No matter how much you will believe it will rain tomorrow, the intensity of your faith can never triumph over the certainty of proven, accurate knowledge. In other words, believing that it would rain tomorrow can never be certain compared to knowing that it would rain. Hence the logical fault in faith.
This is why the belief in a one or more gods, no matter how firm that belief is, always contains at least a grain of uncertanty; otherwise, if you knew and not believed, if you had the knowledge, you wouldn’t have believe, but know, and even saying “I know that [insert a god’s or more gods’ name here] exists”, can be most of the time - if not all of the time - partially uncertain, because of lack of evident knowledge from reliable sources, even if you seem to have your logic allegedly with minimal flaws in it, is within presence. Otherwise, no faith would be required at all, nor intense faith.
The same can be applied to any other mythical creature; they’re popular as illusions, making some think that they are more than illusions, because of how popular they are. Hence the power of tales; hence the fault in the usage of confirmation bias without opening your doubt to other possibilities, leaving you prone to the possibility of delusion. In that sense, mythical creatures exist, but not in the same level as the residents and objects of the Physical world.
I'd like to apologize if I have offended anyone; no such intention existed within my mind.