“Seuss” was his mother’s maiden name as well as his own middle name.
Geisel first used the pen name “Seuss” in college after being removed as the editor of the Dartmouth College’s humor magazine 'Jack-O-Lantern' and being banned from writing for that magazine after he was caught drinking by the dean.
He subsequently started publishing under various pen names, including T. Seuss. and Dr. Theophrastus Seuss, which was shortened to Dr. Seuss. He also had an alternate pen name that he also wrote under which was Theo LeSieg. The “Theo” is short for “Theodor”, and “LeSieg” is “Geisel” spelled backwards.
The proper pronunciation of Seuss is actually “Zoice” (rhymes with “voice”) as it is a Bavarian name. Due to the fact that most Americans pronounced it incorrectly as Soose, Geisel later gave in, stopped correcting people, and decided mispronunciation was a good thing because it is “advantageous for an author of children’s books to be associated with Mother Goose.”
He would have been 108 years old this month. He died in 1991.