The Saturnian Mothership is a Saturnian spacecraft.
When Donald Duck met Saturnian dictator Rebo for the second time, in 1995, he was stationed in orbit of Saturn inside a humongous, semi-organic spaceship. Rebo had converted most of it into a factory line of giant weaponized toys, which he meant to use to exterminate ducks, in revenge for Donald Duck foiling his 1960 attempt to invade Jupiter, before once again attempting to subjugate the Jovians.
However, the Saturnian Saucer sent by Rebo to collect toy models also abducted Donald and his uncle Scrooge by mistake; with them aboard his ship, Rebo diverted his attention to executing them personally. Consequently, when the duo escaped to Jupiter, the Saturnian abandoned his first plan altogether in an attempt to level the planet using the Mothership's heavy artillery.
The Jovians were able to keep the Mothership at bay, but could not prevent it from pelting them with missiles. Thus, they came up with the strategy of using the Virtuo-Beam on Rebo to make him believe that he had accidentally lost his course and was now in front of Earth, and, moreover, make him see the Earth in the Sun's place. Still enraged at the Earthlings, Rebo decided "Earth" was better than nothing and steered the Mothership towards the "planet" at full speed. Though Rebo and his Generals somehow survived crashing into the Sun itself, it is likely that the Mothership itself, and the Toy Army, did not.
Behind the scenes
The Saturnian Mothership appears in the 1995 story The Return of Rebo.