Hexxed Branch

The tree cannot fly, and that is the only thing wrong with it. It hits top speed in about half a second without needing a dash, matches the quickest flying mounts across flat ground, and goes faster still if you launch off a ledge on a diagonal. Summoning it while already falling lets you cut hard turns in midair, and that is the trick that turns it into a boss fight tool rather than a travel toy.

Moon Lord is the obvious target. The True Eyes track you and the deathray sweep punishes anything slow, and simply outpacing the entire pattern along a long flat platform works. Contact damage from the mount is a bonus rather than a plan, since it only fires once you are already at full speed. Fall damage is almost entirely cancelled too, so you can run straight off the end of an arena and keep going.

Master Mode only, from Mourning Wood during the Pumpkin Moon, with the odds improving the deeper into the event you get. In practice that means you need an arena and a build that can already survive the late waves before this is worth farming for.