Just checked the acknowledgements page in The Golden Treasury of Poetry (published by Golden Press) and it says “The Tale of Custard the Dragon” is from Ogden Nash’s The Face is Familiar, with a string of copyright dates ranging from 1931 to 1940.
The Golden Treasury of Poetry is illustrated by Joan Walsh Anglund, and “Custard” wouldn’t have fit on the two page spread in this book with that extra stanza, for what it’s worth.
I’m reminded of the scene from Elf where they discover the children’s book was mistakenly printed with a blank page. “Just ship it, who cares if some kid doesn’t know what happened to the puppy.”
I just bought the illustrated Lynn Munsinger version of Custard and saw the “new” stanza in it. This was my favorite poem as a young child – my mother read it to me dozens of times. Our version in the “Family Treasury of Children’s Stories” by Doubleday did not include that stanza. It was published in 1956. I find the stanza awkward and off-putting. Nash could certainly have made the hypocrisy point more elegantly, so I’m suspicious of how it got there.