Fullback Brent Webb crossed to put Grand Final winners back in front but Horne grabbed his second of the game, although Michael Dobson‘s missed conversion meant Rovers trailed by two points at the interval.
Rovers silenced the Headingley crowd when Kris Welham took advantage of some slack defence to put the visitors in front early in the second period.
But two tries in five minutes from 20-year-old centre Watkins swung the game the Rhinos way.
Rovers reduced the deficit with a try from David Hodgson but Watkins completed his hat-trick from close range.
McGuire stepped past a tiring defence to complete the scoring and seal the win for the Rhinos.
Leeds skipper Kevin Sinfield, making his 400th appearance for the club, finished the game with five conversions.