National Geographic indicates and measures the possibility of forecasting what we will know.  Specifically studies exist that indicate how many different, distinct species exist on our planet but whose characteristics we have yet to classify, identify.

the fish didn’t turn into a fillet—rather, it turned out to be an
undiscovered species. The guide was part of a Conservation
International-led survey of a tropical forest. Scientists rescued the
specimen just in time when they noticed its exceptionally long spines,
likely meant to fend off giant piranhas. Unlike any catfish in reference
books, the as yet unnamed fish was one of 46 candidates for new species
status found within three weeks. “They’re all pieces of a big puzzle,”
says expedition member Philip Willink. “The more we have, the better our
understanding of how the whole world works.”

In the 1730s Swedish botanist Carl Linnaeus set out rules for
classifying species, the most basic biological category: a group of
living things that look alike and normally mate only with each other.
Since then, scientists have cataloged more than 1.7 million. But there’s
still a lot of work to be done. Estimates of the total number of
species range from 3 million to 100 million; one new study puts the
figure at 8.7 million, give or take a million. The numbers at right,
compiled from many surveys and statistical projections, offer the best
guess for species in selected categories.

It occurs to me that this is the other end of the spectrum from “endangered species” on the same continuum. There clearly is not a large population of a given species so that the creature is plainly noticed yet it is difficult to say at first whether there are less of these things than there ought to be.  It exists on a Venn Diagram of classification: it is clearly real and not imaginary yet most of its defining characteristics are speculated upon rather than known.

I suspect I’m being overly poetic.  It also may not be entirely appropriate to mention that this makes me think of Donald Rumsfeld talking about known unknowns.