You know Horton's mantra? Yep, that one: "A person's a person, no matter how small." Well, back in the day, Dr. Seuss threatened to sue a pro-life group for using the line to support their cause. (Source)
Ready for Seuss cross-referencing? In the movie Horton Hears a Who!, there's a scene where a Who is seen holding a plate of green eggs and ham. Very clever, Hollywood (Source.)
Horton made his first appearance in Horton Hatches the Egg. He's pretty cool there, too.