Considering the prominent (albeit controversial) role that scientifically unproven theories about animals have in analysis, particularly translated to reputation and other people's opinions, it doesn't seem unreasonable for creatures that are entirely mythical to appear as viable dæmon forms. Does it?
If scientifically unproven theories (eg. myths, legends, misinterpretations) are to be largely interpreted as belonging to the area of reputation in an analysis for normal animals, does that mean mythical animals are more reputation than fact? Quite possibly so. It doesn't seem unreasonable, however, to point out that reputations are not accurately representative of a person's true qualities, so an analysis entirely composed of reputation really says nothing about the nature of the person it is supposed to reflect, besides that their personality is such that they have this effect on people. That still doesn't really clear anything up - what about motivations? Why are they reacted to in this way? Which of the reputations are true and which are false, when they contradict one another, as reputations almost invariably do in the real world?
So perhaps what we lack in regards to mythic forms is the same flexibility as we have in analysing other animals. Consider animals that have not been studied extensively. As I commented in my musings on the subject here, unstudied species can be analysed in a reasonably objective way by drawing conclusions based on physical traits and the behavioural traits of closely related species. By applying the same method to mythic forms, analysis should be possible.
I'll repeat the example I used in my unstudied forms essay. You give a palaeontologist a skull from an animal he's never seen before, only this time you and I know it to be the skull of an honest-to-God real Western dragon, an illogical fire-breathing six-legged flying reptile. He tells you facts based on the skull (only this time a few things would be a little off because of scientific assumptions; for example, if there were a cavity normally containing an ignition sac to allow flaming breath, the poor limited palaeontologist would probably tell you it was a Jacobson's organ for tasting the air, as many mammals and reptiles have). He can conclude logical things like the size and shape of the brain pan, giving an idea of how intelligent it is, and what the dentation suggests it eats.
The problem here is really that mythics don't always have an agreed prescriptive appearance. If we are to make mythics viable forms, we must be precise about the physical form, because all things "non-physical" like behaviour are, of course, based on various levels of physical structure. Without association areas, higher-order thinking including the personality are not possible, so is the soul a non-physical construct that is free-floating somehow, or is it a physical feature (association areas) causing a behaviour (personality), generally perceived as non-physical? It's an important consideration in mythic analysis.
When it comes to variable species like dragons, the form may be viable as long as it is scientifically picked apart and put together. If the anatomy of the dragon is such and such and such, then in theory, if you passed the dragon you have conceptualised onto another analyser, then similar objective results based on its anatomy should be drawn. This makes it a valid symbol with meaning that is not purely subjective.
But doesn't that seem like a lot of effort? When you are introduced to a self-identified osprey dæmian you think, "Osprey: focussed, direct, a quick learner, sharp". When you are introduced to a self-identified dragon dæmian, however, you cannot identify what the form's characteristics are without further explanation. The fantastic one-word representation system is not adhered to. The necessity of defining your form every time you use it to represent yourself is pointless; you may as well just describe yourself.
I don't count mythics as unviable forms. They are simply difficult. If they are a creature that does not conform to a single phenotype - many different creatures by the same name - then they must be explicitly defined before becoming valid. If they are less complicated, existing in only one real shape, then it's less complicated, but they still must be aggressively analysed for empiricism. For a personality typing system to be valid, after all, it needs to follow a scientific model, and the scientific model used in all other analysis in this system is a zoological one, so if zoological explanations cannot be applied - for example, you need to be able to describe environment, behaviours, diet, life cycle and survival strategies based purely on physical structure, not on unscientific legend - then the symbol is irrelevant to the system being used and the form is not valid to dæmonism as a personality typing system.
Like everyone else to write on this topic, I beg the understanding of mythic dæmians, in that this is my own opinion and is not intended to belittle anyone or suggest that people who choose mythical creatures as forms are in any way inferior to any person not in that category. The only thing I hope to do for any mythic dæmians out there is give them food for thought if they have not already studied this outlook on mythics as forms.