In the comics, Batman & Superman were known as the World's Finest Heroes, having headlined the original World's Finest book for many years. In 1983's "Warpland", the Caped Crusader and the Man of Steel are pulled into another dimension, one where their counterparts are all animals.
I'd like to think Hanna-Barbera & DC were testing the waters for possibly adapting another DC property, Captain Carrot, for television, by trying out characters like Super Frog. Needless to say, Captain Carrot never made it to television at that time.
The image of Superman as an eagle references the Metropolis Marvel's never ending battle for "truth, justice, & the American way", don't you think?