The words "Green Tito, come outside" were uttered once and he was formed.
It is unknown how or why. He just... appeared.
Green Tito was first discovered in a deleted video known as "He came outside." That video was his only visible appearance, but he could only be seen for a frame. However, Green Tito is in every video, we just don't see him.
But he's always there. Watching.
"Green Tito Came Outside And . !" - SuperLuigiLogan.