The episode "Simon Says, 'No Thanksgiving'!" is unique in that it marks the one time that Underdog does not speak in a rhyme when he says "Not plane nor bird nor even frog. It's not even little old me, Underdog. It's a balloon."
The full name of Simon Bar Sinister's henchman is Cad Lackey.
Underdog's lady love Sweet Polly Purebred was based on Marilyn Monroe.
An attempt was made in the early 90's to re-release 'Underdog' in syndication but because of the drug culture at the time all references to the "secret energy pill" were removed. Without them the cartoons did not make sense and were quickly pulled. This is a little ironic since the original reason the pills were added was to encourage children to take their vitamins.
In their book "How Underdog Was Born...", W. Watts Biggers and Chad Strover reveal that seeing Wally Cox's performance in the 1963 movie Spencer's Mountain (1963) inspired them to ask him to voice their newly created character, Underdog.
When the series was brought to Japan in 1967, it was called "Urutora Wan-chan" ("Ultra-Puppy"), due to the huge popularity of "Urutoraman: Kûsô tokusatsu shirîzu" (1966) and "Urutora sebun" (1967) at the time, and because "Ultra" cleverly fit the "U" on Underdog's chest.
The Underdog theme song lyrics have almost the exact same meter as the poem "Excelsior" by Henry Wadsworth Longfellow. As a result, you can sing the words of the poem to the theme.
Underdog's character became so popular that it appeared as one of the giant balloon's in Macy's Thanksgiving Day Parade on November 25, 1965, barely one year after the program's debut. It's last appearance in the parade was in 1984.
In the opening where the lines: "It's a bird, it's a plane, it's a FROG" are uttered, the woman who says "frog" is sporting a campaign button that says "Barry" on it, referring to senator Barry Goldwater who was running for president in 1964.
The "Simon Says, No Thanksgiving" episodes were created to honor the first appearance of the Underdog balloon in Macy's Thanksgiving Day Parade. They aired at noon on November 25, 1965 immediately following the event.