Thunder Guppy. Ha ha, good one! In my neck of the woods the term "guppy" is almost exclusively used by the engineers.
Fluff, we were told, says Fat Little Ugly Flying F****r. Not "fellow" as far as I know. This applies to the -200 only. The -300/400/500 are known as "classics", or alternatively just by their numerical moniker, as it should be.
I heard the Germans called the -200 "ze tin mouse". Seen from the cockpit of a 747...
Happy landings!