Anal is a variant of her current scene. During modifier selection, you can pick it by pressing the "anal" button just to the upper right.
(You need to complete anal in her base scene once before getting access to it in Sandbox Mode)
Viewing post in SpunkStock: Music Festival v0.12 Demo comments
Fufu is the only character with an "anal" option. Some other characters have other options for "variants" though. like Ayra having a facesit, and certain boss characters having intensity variants.
There is technically 1 other character with an Anal scene, and that is the femboy, Secret. (He is not in this demo).