There were five former presidents alive during at least a part of the Civil War: Martin Van Buren (d. 1862), John Tyler (d. 1862), Millard Fillmore (d. 1874), Franklin Pierce (d. 1869), and James Buchanan (d. 1868). Did any of them express opinions about the war publicly or privately? Did any of them show support for the Confederacy over the Union? How did they react to Lincoln's assassination?
There's definitely more that can be said, but there've been several previous good responses to this question: this one by /u/secesionisillegal, another by /u/lord_mayor_of_reddit, and still another by /u/chaingunfighter. Also, this related answer by /u/Georgy_K_Zhukhov talks more about President Tyler specifically.