Old MacDonald is a character referenced and/or mentioned in the Shrek series. He is from the nursery rhyme "Old MacDonald Had a Farm".
Old MacDonald is mentioned in one of the good deeds in "Mother Goose Land", giving Shrek the task to fart on the Cow that Jumped over the Moon. Otherwise, he is not seen in-game.
He does however appear in the Gamecube port where he is seen as an NPC.
Shrek Forever After
Old MacDonald is mentioned by Donkey as one of his former bosses. According to Donkey he put him in pretty bad working conditions, constantly telling him to say "e-i-e-i-o".