They are only found in perfectly circular lakes, hence they are very elusive. But if you do see one, it is said that you should immediately make an attempt to capture one. Catching one is tricky, but it is doable.
To capture, row to the exact center of the lake and bore a square hole in the water. Bait the hole with cheese. The best types of cheese that will attract the Whiffenpoof sooner are Stilton, Brie, Liederkranz, or Limburger. Usually, the Whiffenpoof will smell it quickly if you use the listed cheeses, and will come to the surface to feast. When it emerges, squirt tobacco juice its eye. This will make the creature get so angry that it will not go back into the lake, so you can then easily net it.