I imagine 600 years ago people would constantly ran into creatures that they had never been educated about before. Creatures that had never been documented before. Whether it be a creature of land, sea or air.

And how would they describe what these animals were? I'm sure often they would just say they saw a dragon. A monster, etc...

This seems to be what alien UFOs are today. When something is seen in the sky. They are assigned the title of whatever the observer wishes to assign to it. Still, quite often we see people reporting UFOs (but not nearly as much as you'd expect with the amount of cel phones cameras video cameras, space exploration equipment)

Still, no matter the extent of the observer's imagination, these UFOs remain to be exactly what their acronym implies... UNIDENTIFIED.