The horizon is a circle that divideth part of the world seen from the part that cannot be seen.
– The New Book of Knowledge (1767)
The image of the fisherman comes from a grammar book I found at American Antiquarian Society called A New Guide to the English Tongue, published in Philadelphia in 1770. This picture was used to illustrate a fable called "The Fisherman and the Fish" in which a captured fish asks to be released, promising that she will come back and allow herself to be caught when she's grown larger. The fisherman says no, and the moral of the fable is "Never let go a Certainty for an Uncertainty."
The sea monster is a European image, from the mid-1600s.
We tend to believe that every generation expands its horizon line of knowing a little more, when in fact the unknown remains always on the horizon. Hopefully, as Rilke said, "everything that frightens us is, in its deepest essence, something helpless that wants our love." Or at least it wants our attention.