Stegosaurids had small brains by Dinosaurian standards, although not so small as to fall below the average range of reptilian brain to body weight ratio (the dim-witted dinosaur - all brawn and no brain is in fact a myth).  Stegosaurus is famous for its "second brain" - actually an enlarged pelvic nerve plexus situated in the hip region.  In fact all dinosaurs and birds and many other Sauropsids have such organs (so do mammals, but not developed to such an extent).
      This figure shows the Kentrosaurus brain (a) next to the much larger and pelvic nerve ganglion (b)