Science doesn't know everything, but it sure knows a hell of a lot more than it used to. Back in what we will collectively term The Olden Days, people simply didn't have the kinds of incredible resources that we do now.
Considering that we can now do things like photograph the double helix of DNA and peer into the earliest universe, we have a bit of an unfair advantage over our scientific predecessors, who would pitch themselves head first into the world of scientific enquiry armed with nothing more that a magnifying glass and a notebook.
That said, they still had some pretty frigging weird ideas as to how the world worked.
As the world got bigger and horizons got pushed back, all of the weird and wonderful things in it became known to a wider and wider community. After all, if elephants could have an extra limb growing out of their face, why wouldn't all manner of fantastical beasts also exist?
And given that the scientific method mainly consisted of just sitting down and having a good old think about something, it's no wonder that their theories were sometimes a little...strange.