Possible Clues:

• Celtic sea god
• The Once-__: "The Lorax" character
• Celtic Neptune
• The Once-___ ('The Lorax' narrator)
• The Once-___ (antagonist in 'The Lorax')
• The Once- -- ('The Lorax' character)
• Celtic water deity
• The Once-___ (Seuss character)
• Celtic god of the sea
• Actor Burton