The Ark is a giant airship owned by Sailor Venus' past self.
Sailor Moon: Another Story
The Ark located at the village of Sariel. It is where Sailor Venus and Ishtar battle. Later, the Sailor Senshi travel back to the Silver Millennium and found the Ark and activate it, allowing them to travel the world quickly in their search for Sin.
Sailor Moon Crystal: Another Story
Sailor Moon Crystal: Adventures in FernGully
The Ark appeared in Sailor Moon Crystal: Adventures in FernGully. The Ark used Crystals and Gems from Crash Bandicoot as fuel and the seven Rainbow Gems from Castle of Illusion starring Mickey Mouse act as a source of fuel for the Ark, similar to how the Comet Observatory and the Starship Mario used Power Stars as fuel in Super Mario Galaxy and Super Mario Galaxy 2, respectively and how the Odyssey used Power Moons and Multi Moons as a source of fuel in Super Mario Odyssey. Each Rainbow Gem is collected, the Ark's sail changed color. Eventually, the sails colored rainbow with all seven Rainbow Gems collected.