Your Saturday Special

Happy Saturday, everybody. Are you ready to be frustrated and fuzzled? Okay, maybe not… but I do have a nice little riddle ready for you. I doubt it will take the “riddle champions” long to figure it out, so everybody else hurry and get your guess in.
Today’s riddle subject exists in the real world, so get Google fired up.

You are looking for a thing.
We know of this thing as a single object, but actually it is two objects.
This was probably known to ancient man.

It was definitely known by the late 16th century.
Sometimes you see it, sometimes you don’t.
This object was the first of its kind found.

It was wonderful, and astonishing.
An entire category of wonderful and astonishing things were named after this.
Old Library Card Catalogue File
Have you any good ideas about what this critter might be? You know where to find me…
DON'T LOOK, TRUDY!