Viper's Sprouting Mount

You touch a medium or smaller beast, that has no fly speed or swim speed and a challenge rating of 1/2 or lower, and magically enlarge it to a perfect riding size. The creature's size is now large and it has a speed of 60 feet, if it doesn't already. The creature's hit points is now 45, if it isn't already,  and when it reverts to its normal form, the creature returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form. As long as the excess damage doesn't reduce the creature's normal form to 0 hit points, it isn't knocked unconscious. You can use the same riding rules as the riding horse for this new mount.