Old McGee is a rather plain looking enemy. He has long green arms with a roundish body on the top, and a flat body on the bottom.
Old McGee is in the cage where Catherine Derigueur has him captured with Mule McGee and Headcase. After you have defeated her, Old McGee, along with Mule McGee will come out of the cage and try to attack you. All you have to do is hit both of them once and you will get two secret bonus points. However, if you strike too late, Old McGee and Mule McGee will leave the screen on the left where you can't go anymore and you will miss out on the secret bonus points.