Agamenticus—a twin-screw, double-turreted ironclad monitor—was laid down sometime in 1862 at the Portsmouth (N.H.) Navy Yard and launched on 19 March 1863. Since operational experience with the monitors during the Civil War had shown the necessity for better ship-control and navigational facilities, Agamenticus underwent alterations in the first few months of 1864, notably the addition of a "hurricane deck" that extended between the two turrets and over the machinery spaces amidships.
Commissioned on 5 May 1864 at Portsmouth, Lt. Comdr. C. H. Cushman in command, Agamenticus operated off the northeast coast of the United States, from Maine to Massachusetts, until decommissioned at the Boston Navy Yard on 30 September 1865. She remained laid-up for nearly five years and, during that time, on 15 June 1869, was renamed Terror (q.v.).