Possible Clues:

• Star of a Seuss book
• Seuss character who "speaks for the trees"
• Seuss character who 'speaks for the trees'
• Seuss activist
• Ecology-minded Seuss character
• Seuss's environmental crusader
• Hit 2012 animated film, with 'The'
• Dr. Seuss character
• Dr. Seuss character, with "the"
• Character voiced by Danny DeVito, with "The"