Possible Clues:

• Ecological character voiced by Danny DeVito, with 'The'
• His story is told by the Once-ler
• Ecological Seuss character, with 'The'
• Seuss book with environmental undertones, with 'The'
• Seuss's environmental advocate
• Ecological Seuss character, with "The"
• Environmentalist in a Dr. Seuss story
• Seuss's nature spokesman
• "I speak for the trees" speaker
• Kid's character who ''speaks for the trees''