But "Horton Hears A Who" exists also in the creative universe of musical theatre, as one of the main plots in "Seussical!". It was made into a very stylized short film in Russia using animation painted on glass. And it became part of the Tooniverse in 1970, thanks to the legendary Chuck Jones (who brought Theodore "Dr. Seuss" Geisel's character of the Grinch to Television a few years earlier. Most of the voices were supplied by Hans Conreid (Waldo Wigglesworth!), June Foray (Rocket J. Squirrel!), Thurl Ravenscroft (Tony the Tiger!), and Chuck Jones himself.
In the Tooniverse, the world of the Whos is contained in a dandelion puff. In the movie version of "How The Grinch Stole Christmas", it's contained in a snowflake. I'm not prepared to say that the characters in 'The Young Ones' could also be Whos, but it's pozz'ble, just pozz'ble, that there could be thousands of Who-worlds to be found floating in our atmosphere among the dust and the pollen.....
The story of "Horton Hears A Who" may have been inspired by the McCarthy-Army hearings in the 1950s, and its basic moral of "A person's a person, no matter how small" has been hijacked by right-to-life groups over the years. (Something which Dr. Seuss wanted to fight in the courts, apparently.)
Here's a basic summation of the story, from my favorite source, Wikipedia:
The book tells the story of Horton the Elephant who, on the fifteenth of May in the Jungle of Nool, hears a small speck of dust talking to him. It turns out the speck of dust is actually a tiny planet, home to a city called "Who-ville", inhabited by microscopic-sized inhabitants known as Whos.
The Whos ask Horton (who, though he cannot see them, is able to hear them quite well) to protect them from harm, to which Horton happily obliges, proclaiming throughout the book that "a person's a person, no matter how small". In doing so he is ridiculed and forced into a cage by the other animals in the jungle for believing in something that they are unable to see or hear.
His chief tormentors are Vlad Vladikoff, the Wickersham Brothers and the Sour Kangaroo, and the small kangaroo in her pouch. Horton tells the Whos that they need to make themselves heard to the other animals, lest they end up as part of "beezlenut stew", which they finally accomplish. The Whos finally make themselves heard by ensuring that all members of their society play their part.
In the end it is a "very small shirker named Jo-Jo" whose final addition to the volume creates enough lift for the jungle to hear the sound,thus reinforcing the moral of "a person's a person no matter how small."
Now convinced of the Whos' existence, Horton's neighbors vow to help him protect the tiny community.
As to whether or not the main Toobworld has its own version of Whoville, that hasn't been established yet so far as I can tell. 'The Young Ones', grotesque as they may look at times, hardly resemble the Whos of Dr. Seuss.....