The legend of the Black Cat (AKA the Demon Cat) is shared by the White House and Capitol Building, a few blocks away.

At the White House, the Black Cat is seen in the basement before various tragic events.  But up in the Capitol, it apparently roams the halls at will. It should be noted that back in the 19th century, both buildings employed cats to check the rat population, which is numerous in Washington.

Supposedly (No actual report exists) A Capital Building Policeman (The Capital has its own police force, as does the US Supreme Court and the local DC federally managed park system) said he saw the cat in the very early 19th century and another was said to have shot at it in 1862. “It seemed to grow” he said “as I looked at it. When I shot at the critter, it jumped right over my head”

The cat sightings in both the White House and Capitol Building tend to follow a national tragedy.  A White House guard claimed to have seen just before the Lincoln assassination, a week before the stock market crash of 1929 and also reportedly seen days before the assassination of JFK. The last semi-official sighting of the Demon Cat was in 1940.

Interestingly enough, a few block away from the White House sits the Octagon House, which is said to be curse and haunted.  Legend says that Betty Taylor, the married niece of the first owner of the house, tripped and fell to her death by a black cat as she raced down the houses circular stairs. She was running in the dark to greet her lover who entered the property by a secret passage that opened on the bank of the Potomac (The river has since been pushed by, but at one time it did run close to the house)