The Once-ler from Dr. Seuss' The Lorax is Revealed At Last

In Dr. Seuss' book The Lorax, it costs the boy 15 cents, a nail, and the shell of a great, great, great grandfather snail to hear the story of the Once-ler.
Suess never revealed who ... or what the Once-ler was, but soon, you'll be able to see the face of the Thneed industrialist, courtesy of Hollywood:

That elderly, decrepit fellow was similarly depicted in the original Seuss illustrations as a pair of eyes between the slats of a boarded up window, and those scenes in the story provided the single strongest piece of evidence that the seemingly furry-limbed villain was human.
Actually, it was his Snuvv that gave it away.
Snuvv?
This rhyme explains it (as much as any Seussian nonsense verse can.) It accompanied a drawing — seen below — of the Once-ler accepting payment from the boy before telling his tale of woe.
Then he hides what you paid him
away in his Snuvv,
his secret strange hole
in his gruvvulous glove.
“If there was a clear sign this character was something other than human, we would have abided by that,” says Meledandri. “But okay, he’s wearing gloves. You’re not going to put gloves on a monster.”

The Lorax producer Christopher Meledandri went on to explain the philosophical underpinning of making the Once-ler a man instead of a monster. Read the entire story at Entertainment Weekly's Inside Movies: here.