The Yellowstone-class runabout was an advanced prototype designed by Harry Kim in an alternate reality in which he never joined Voyager, and which he stole to return to his own timeline.
The runabout appears in the Voyager episode Non Sequitur. After a shuttle accident in a time stream, Harry Kim wakes in San Francisco in a version of 2372 where he stayed on Earth and became a designer at Starfleet Engineering. There he and his colleague Lieutenant Lasca have developed the USS Yellowstone, the first of a new class of runabout with improved tetryon plasma engines.
Kim realises that he does not belong there, and Starfleet, finding gaps in his story, suspects him of spying for the Maquis. With the help of that reality's Tom Paris he breaks out of custody and takes the prototype from its orbital dock.
Pursued by a Starfleet ship, the two recreate the conditions of the accident. Paris stays aboard as the Yellowstone is destroyed, and Kim returns to Voyager.