What is Seabiscuits family tree?

Seabiscuit

b. 1933 d. 1947

Seabiscuit's sire:Hard Tack

Seabiscuit's dam:Swing On

Hard Tack's sire:Man-O-War

Hard Tack's dam:Tea Biscuit

Swing On's sire:Whisk Broom II

Swing On's dam:Balance

If you want a more detailed pedigree Wikipedia has I believe a five generation chart. You can also try Jockey Club, the American Thoroughbred registry. If the horse was born in America they will have a record of it.