The natural history and illustration of each beast was usually accompanied by a moral lesson. This reflected the belief that the world itself was the Word of God, and that every living thing had its own special meaning. For example, the pelican, which was believed to tear open its breast to bring its young to life with its own blood, was a living representation of Jesus. The bestiary, then, is also a reference to the symbolic language of animals in Western Christian art and literature.
Bestiaries were particularly popular in England and France around the 12th century and were mainly compilations of earlier texts. The earliest bestiary in the form in which it was later popularized was an anonymous 2nd century Greek volume called the Physiologus, which itself summarized ancient knowledge and wisdom about animals in the writings of classical authors such as Aristotle's Historia Animalium and various works by Herodotus, Pliny the Elder, Solinus, Aelian and other naturalists.
Following the Physiologus, Saint Isidore of Seville (Book XII of the Etymologiae) and Saint Ambrose expanded the religious message with reference to passages from the Bible and the Septuagint. They and other authors freely expanded or modified pre-existing models, constantly refining the moral content without interest or access to much more detail regarding the factual content. Nevertheless, the often fanciful accounts of these beasts were widely read and generally believed to be true. A few observations found in bestiaries, such as the migration of birds, were discounted by the natural philosophers of later centuries, only to be rediscovered in the modern scientific era.
Two illuminated Psalters, the Queen Mary Psalter (British Library Ms. Royal 2B, vii) and the Isabelle Psalter (State Library, Munich), contain full Bestiary cycles. That in the Queen Mary Psalter is in the "marginal" decorations that occupy about the bottom quarter of the page, and are unusually extensive and coherent in this work. In fact the bestiary has been expanded beyond the source in the Norman bestiary of Guillaume le Clerc to ninety animals. Some are placed in the text to make correspondences with the psalm they are illustrating. 
The Italian artist Leonardo da Vinci also made his own bestiary.
The Aberdeen Bestiary is one of the best known of over 50 manuscript bestiaries surviving today.
Mediaeval bestiaries are remarkably similar in sequence of the animals of which they treat.
In modern times, artists such as Henri de Toulouse-Lautrec and Saul Steinberg have produced their own bestiaries. Jorge Luis Borges wrote a contemporary bestiary of sorts, the Book of Imaginary Beings, which collects imaginary beasts from bestiaries and fiction. Nicholas Christopher wrote a literary novel called "The Bestiary" (Dial, 2007) that describes a lonely young man's efforts to track down the world's most complete bestiary. Writers of Fantasy fiction draw heavily from the fanciful beasts described in mythology, fairy tales, and bestiaries. The "worlds" created in Fantasy fiction can be said to have their own bestiaries. Similarly, authors of fantasy role-playing games sometimes compile bestiaries as references, such as the Monster Manual for Dungeons & Dragons. It is not uncommon for video games with a large variety of enemies (especially RPGs) to include a bestiary of sorts. This usually takes the form of a list of enemies and a short description (e.g. the Metroid Prime and Castlevania games, as well as Dark Cloud and Final Fantasy).
Some mythical creatures, such as the dragon and griffin, have their origin in traditional mythology and have been believed to be real creatures. Others were based on real baby creatures, originating in garbled accounts of travelers' tales; such as the Vegetable Lamb of Tartary, which supposedly grew tethered to the earth (and was actually a type of fern). Examples of the legendary creatures can be found in medieval bestiaries.
Conversely, some creatures downplayed as just storytelling, have been rediscovered and found to be real in recent books, such as the giant squid (the Kraken). In Africa, Natives of the Congo told European visitors of an animal that looked like a cross between a zebra and a giraffe. While the visitors assumed the stories were just folk tales, in 1901, Sir Harry Johnston brought back pelts that proved the creature, which we now call the okapi, was real.
Often mythical creatures are hybrids, a combination of two or more animals. For example, a centaur is a combination of a man and horse, a minotaur of a man and bull, and the mermaid, half woman and half fish. These were not always intended to be understood as literal juxtapositions of parts from disparate species. Lacking a common morphological vocabulary, classical and medieval scholars and travelers would attempt to describe unusual animals by comparing them point-for-point with familiar: the giraffe, for example, was called camelopard, and thought of as a creature half-camel, and half-leopard. The leopard itself was so named as it was historically believed to be a half-lion (Latin: "leo") and half-panther (Latin: "pardus"). This etymology has been kept until the present day, despite its zoological inaccuracies.
Many legendary creatures appear prominently in fantasy fiction. These creatures are often claimed to have supernatural powers or knowledge or to guard some object of great value, which becomes critical to the plot of the story in which it is found. Dragons, for instance, are commonly depicted as perched on a gleaming hoard of gold which becomes the target of adventurers.
Other legendary creatures are thought to exist even today, including chupacabras, Bigfoot, Yeti, the Loch Ness Monster, the Jersey Devil, Mothman, Yowie, Yeren, Pukwudgie, and even space aliens. These are called cryptids by cryptozoologists.
Throughout history legendary creatures have been incorporated into heraldry and architectural decoration. Legendary creatures have also been accepted into many facets of popular culture, most notably in fantasy role playing games such as Dungeons & Dragons or Everquest, video games, and Hollywood movies.