Stallman liked GNU because it's unpronounceable. When Europeans first discovered the animal, Africans referred to the first sound in its name as a click, which the Europeans didn't use in their languages, so the "g" in the word essentially stands for "unpronounceable sound," and is usually omitted. He used it for fun but brought back the sound in the form of a hard "g."

"Gnu" is a real word in the dictionary.

Edit: On my system it's the only ^.nu$ in the dictionary.