The Sealion was a prototype Missile Boat delivered to the Rebel Alliance.
The Sealion was a Missile Boat prototype, that was stolen by forces under the command of Rogue Grand Admiral Zaarin. When Zaarin conferred to his officers about the second Death Star and that the Rebels were actually walking into a trap at Endor but didn't intend to warn them, one of his loyal commanders Namuura Din who lead the Victory-class Star Destroyer Zeplin decided to take the information and the Sealion and defect to the Alliance. While Zaarin was unable to stop him, Loyalist forces led by Darth Vader in the Imperial-class Star Destroyer Garrett managed to intercept and capture Din's flagship, but by the time they arrived the Sealion was already transferred to the Mon Calamari Cruiser Link and the cruiser jumped to lightspeed.
The commander of the Link attempted to get the news to Rebel Command, but the ships engines overloaded forcing the ship out of Hyperspace. The Links commander sent the Sealion to a Rebel stronghold, a XQ4 Platform Enin'sd to be fully analyzed. But the Rebels never got the chance to disassemble the vessel and learn its secrets before the Imperials arrived at the base with the intention of either destroying or otherwise illicitly recovering the Sealion, as the Prophets of the Dark Side discovered when recovering it illicitly before its container was destroyed, preventing any chance of the Rebels making their own Missile Boats. Nonetheless, the Rebel's capture of the Sealion, combined with the treacherous Grand Admiral Zaarin's breakaway faction's destruction of the Missile Boat's production facilities via planetary bombardment, led to the suspension of all Missile Boats from service until further notice at Emperor Palpatine's order, with all remaining Missile Boats being placed into a secure holding area.
The Sealion only appears in Star Wars: TIE Fighter Battle 13 Combat 4. The first mission regarding the prototype shows the ship on the Link and the third and final mission about the fighter hiding in one of the Class-B cargo container group Niicee.
There are two methods of handling the Sealion. The primary objective is to destroy the container, and with it, the Sealion. The secondary objective in the game is to recapture Sealion at the Rebel Base to which it flees. The player has to identify the container holding it and allow a cargo ferry to retrieve it.