From Muppet Wiki

Old MacDonald had a farm. He has appeared on Sesame Street many times over the years, beginning in the early 80s, and often performed by either Richard Hunt (early appearances) or Martin P. Robinson. He usually has a slight Scots accent, and appeared in two Sesame Street News Flash sketches in which Kermit the Frog investigates events on the farm.

Old MacDonald has also introduced music videos by The Oinker Sisters and occasionally been involved in street storylines. On one such occasion, Shelly the Turtle wanted to join his farm menagerie, and be added to the song. Most recently, the old farmer appeared with Gladys the Cow in Season 36 to promote healthy eating.