The Celestia is a massive pre-war battle-ship that was part of Equestria's royal navy. The ship was constructed during the war and was likely used by Equestria in various battles.
The Celestia was built during the Equestrian - Zebra conflict that engulfed the two nations. The ship was built along with its sister ship The Luna which dwarfed the Celestia, being so massive it took well over a century after the Last Day to sink. The Celestia ran aground during the last day, becoming partially sunk, but not completely.
The Celestia was made into a Steel Ranger outpost and base of operations. The battle-ship had weapons and technology that were considered extremely rare and valuable by the Rangers and worth protecting. The Steel Rangers stored various supplies and other rare technology aboard The Celestia. The Celestia's massive cannons, required shells which the ship lacked, fortunately for the Rangers, the nearby Luna had more than enough munitions.
The Celestia was used to fire upon the Hoofington Hoofball Dome, the base of operations for the Reapers during the Reaper - Steel Ranger war. The ship and all its technology were lost in the bay were it was moored when a blind and heavily injured Blackjack fired Trottenheimer's Folly into the ship hull, causing the vessel to sink. What was left of the vessel was destroyed and looted by the Reapers and their allies.