I'm ewe. (Credit to Gavon for that one.)
The horse with the long face says "In the First Amendment, I'm a horse and even I knew that."
Because she's afraid she might fly off the handle.
Because chickens didn't exist yet, even though Christine O'Donnell thinks they did.
Christine O' Donnell replies, "Doc, I know it's a sin, but is it hurting my vision?" The doctor says, "No, but you're upsetting the other patients in the waiting room." (Credit to Tanner for that one.)