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.