How did this three-legged man work?

I think it's small and on the side of his calf/knee based on this pic https://images.findagrave.com/photos250/photos/2006/15/12891634_113745349928.jpg

Doctor #1: What's that?
Doctor #2: I don't know, a foot maybe?
Doctor #1: meh, good enough, 3 legs, four feet

He had a parasitic twin joined at the basal spine, and had the twin's leg, with a club foot coming out of the knee - hence four feet and three legs.

This just made me think of the thing from "inside"