Stegosaurus

“This archival image, snapped circa 1899, depicts paleoartist Charles Knight working on a scale model of the dinosaur Stegosaurus. In life, this species could measure 28 ft (8.5 m) long and weigh about 6,000 lbs (2,720 kg). But when this animal was discovered, paleontologists were surprised to find that its skull—and brain—were disproportionately small. In fact, some scientists thought this massive herbivore must have had a "second brain" near its hips that controlled the back half of its body. Turns out, Stegosaurus did manage with just one relatively small brain.“

(source 1 & 2)