In the funny paper.

"There was the invisible dog of the patrick Bertigrew Bagley, who was more ape than dog, and who could sometimes be seen if one knew how to look. Foley saw him now, and the plappergeist winked solemnly at him. Freddy knew who he was then. He was the island-ape who used to be in the Katzenjammer Kids in the funny paper. But all grotesque funny paper characters have independent and exterior existence, unknown usually to their drawers. It was good to have the dog, the ape, the polter-plappergeist on your side. He was smarter and more mischievous than other dogs or apes, and he could kill effectively." 

-from Fourth Mansions

Over 100 years later, it's still hard to find Katzenjammer Kids comics.

Some future fan/scholar will have to do the hard work of digging through thousands of microfiche newspapers. What's microfiche? What's a newspaper?

I've become increasingly convinced (after reading My Heart Leaps Up) that early 20th century popular culture (nostalgic trash) is one major key to one major Lafferty door. Anyone annotating a Lafferty book better brush up on newspaper strips and big bands. A quick search reveals only one candidate for a Katzenjammer island ape, and this is only preserved in a Turkish translation!

http://kayaozkaracalar.blogspot.com/2011/05/turkish-debut-of-katzenjammer-kids-1937.html

This may or may not be the right ape. Perhaps future Lafferty scholars (or Katzenjammer scholars) will someday make the identification.

I've often thought that there's an interesting essay to be written by someone willing to wrestle with all of Laff's references to comics, uses of comics in his stories, and sometimes comics-logic. Alas, I don't think that I'll ever write that essay, but I'll be first in line to read it if someone else takes up the challenge!