Which American Thoroughbred horse won the triple crown in 1977?

The correct answer was Seattle Slew.

The American Thoroughbred race horse, won the Tripple Crown in 1977. He was tenth of thirteen horses to accomplish the feat. He is also one of two horses to have won the Tripple Crown while having been undefeated in any previous race. He was also honored in 1977 as Horse of the Year.