Mystical Creatures are non-human life forms with a strong magical nature. Many of them can interbreed with humans and produce variable fractals. They can be devided in to three camps: true-mystical, non-sentient, and pseudo-mundane.
The truly mystical creatures include:
True-mystical creatures are defined by their magical nature, and their inability to exist without magic. All possess innate magical ability and many can shape-shift. Dragons are the most infamous, though unicorns come as a close second. While a dragon will not outright die if all magic is removed from an area, they will be severely weakened.
The Pseudo-Mundanes are called this because, while they are magical in nature and mystical in origin (as opposed to artificially-created or "metal" creatures), they have a lifecycle and ways that are not governed by magic. Aten, for example, build spacecraft, while centaurs have been known to hold office and dwell in otherwise human cities. These beings do not shape-shift or benefit from innate magical abilities. They may have Magic of their own, but it must be learned like human magic.
Non-sentient creatures lack intelligence. They may possess innate magic but act primarily on instinct rather than thought. What seperates them from regular animals is their life-cycle. Basilisk, for example, "feed" by turning living creatures into stone. Galgan are effectively very large arachnids, and lack even a fully-developed brain. Kraken are in fact simply giant squid, with all of the same intelligence; but use innate magic to support their large size and gain a significant fraction of their energy requirements through magic (as opposed to eating).
Little is known about mystical creatures during Antiquity, though Dragons are presumed to have been teachers to humans during that time. There is also evidence that all the major species fought on one side or another, or sometimes both, of the Mythical Age.
Becuase no human records persist before the Age of Darkness, knowledge of Mystical Creatures began during that time. This is when the term was coined and most interactions began. The legends and myths about the different mystical creatures probably evolved during this time.
By the Mage Wars, many types of Mystical Creatures were actively hunted for different pieces of their anatomy, used as ritual components in spellcasting. In addition to causing most species of mystical creatures to fear and avoid humans, this had a significant impact on their numbers.
By the end of the Mage Wars, most populations of mystical creatures had withdrawn to less human-populated portions of the verse, with some species becoming so rare they faded into myth.
The exception, of course, was the dragons, who remained visible and protective of their lands.
The standard mystical creature types were established by the Gudersnipe Foundation and first published in the early Golden Age, but were based on extant manuals in regular use by the old Gudersnipe Army during the Second Chaotic Period of the Mage Wars. The underlying research to typify them has never been released for general review, and has caused numerous disagreements.
Dragons, for example, have a Centaur-form they can take, yet centaurs are not considered in the same class as the rest of the dragon forms. However, mystical creature typography is not based on dragonology.