Creatures depicted in heraldry that are not known to have existed as actual animals are frequently categorized as monsters. While some of these creatures have no basis in reality at all, some are based at least partially on some animal that does or did at one time exist.
For example, the image of the unicorn is said to have come about as a result of observations of wild antelope in Syria and Palestine. When viewed from the side, the two long horns of these animals gave the illusion of a single horn. Thus, the image of the unicorn was born.
centaur (a centaur with bow and arrow may be termed a sagittarius)
(a mermaid with two tails is called a melusine)
wyvern wings displayed