The creature itself is so ugly, that it avoids bodies of water on nights of a full moon so as to not have to see its own reflection. If it does see its own reflection, it will cry, potentially dissolving itself with its tears. (Same, says Grace).
It is known to roam the Hemlock forests of NEPA, so if you happen to be out in the woods there keep an eye (and ear) out.
A Squonk-based song for your entertainment: