With John Sebastian, words and phrases were meant to be metaphors and pictures. I guess just like everybody elses lyrics were and are. I have never heard of a "Sleepy Bull Toad" before I heard this song but I knew what he meant when I heard it. I have never seen a Buttermilk Sky but I can imagine one.
There is a lot of Jug Band in whatever Sebastian did. Before The Lovin' Spoonful he was in The Even Dozen Jug Band and he played with some of the best of the Jug Band rejuvenators of the 60's. A little country color was bound to sneak into his lyrics o matter what he was writing.