Santa's Scout is a home-made doll made by Susan when she was a child. It only appeared in Cabin Fever.
A few days before Christmas, Susan told Greg, Rodrick, and Manny that Santa's Scout was an elf that spies on the kids and goes back to the North Pole and tells Santa Claus about how the kids behaved.
Rodrick started feeding it all bad things Greg did, and just then, Rodrick found out it wasn't real. To scare Greg, he kept moving it everywhere. This started creeping Greg out and he would behave his best whenever Santa's Scout was there, believing that it was real. Also, when he saw that there wasn't any presents under the Christmas tree, he thought the Santa's Scout had told on him.