Scientists used to think that Stegosaurus had a brain in its butt.

This was because it was hard to believe that the 24 foot long animal could have survived with a brain roughly the size of a walnut. This led to suspicion that the stegosaurus was so large it needed two brains to maintain its bodily functions. Part of this confusion stemmed from the fact that the stegosaurus’ spinal cord features a suspicious bulge near its rear end. This enlargement of the spinal cord in the hip area was actually its sacral plexus, an intersection of various nerves that serves the animal’s pelvis, back legs, buttocks, and genitals. None of this conspicuous nerve bulge was made up of brain matter.

Naturally, despite the fact that scientists later disproved the “second brain in its rear end” theory, it was too good for the press to let go. The theory was featured in a popular column in the Chicago Tribune in 1912, and nearly 100 years later, it hasn’t escaped the collective consciousness.

Read more about the 2-brain theory at the Straight Dope. And you can get all of your stegosaurus facts at How Stuff Works.