Per the Star Wars: The Rise of Skywalker: The Visual Dictionary: the ship is named the Night Buzzard.
The Night Buzzard was a modified Oubliette-class Transport used by the Knights of Ren during the First Order–Resistance war.
The Buzzard was originally a dungeon ship on Osseriton, a penal world in the Unknown Regions. The vessel was liberated by the Knights during a raid on the planet alongside many prisoners.
The Knights made several alterations to the Night Buzzard to better suit their tastes, such as altering the vessel's appearance to match their dark side aesthetic. The vessel's engines were also crudely modified to generate more thrust, although this took a toll on the insulation baffles, causing the Buzzard to spew noxious gas as it flew.
A popular Star Wars YouTube channel also published this information yesterday based on the Visual Dictionary:
The Knights of Ren - Everything We Know So Far