The 'true' ruling planet for Scorpio is (and always has been) Mars.
Pluto was given 'rulership', following its discovery in 1930, by a bunch of misguided Astrologers, who clearly had no real understanding of the symbolic nature, and structure, of Astrology. Unfortunately, most New Age Astrologers accept this spurious Plutonian rulership of Scorpio as a matter of course. Many believed (and probably still believe) that two more planets would be discovered; one to replace the Venusian rulership of Libra, and the other, the Mercurial rulership of Virgo. Some, jumped ahead and began using the minor planetoid Chiron as 'ruler' of Virgo. Whilst others began cluttering their charts with all manner of Asteroids, Minor Planets, and any other bits of space-gravel that took their fancy.
In Classical Astrology*, each planet (except for the Luminaries) holds rulership over two signs (for symbolic reasons far too boring and esoteric to explain here). The associations of Uranus with Aquarius, Neptune with Pisces, and Pluto with Scorpio, are some of the main reasons why Modern Astrology is such a load of bollocks.
* ie, the type of Western Astrology practised for thousands of years prior to the discover of Uranus, Neptune and Pluto, which sent pseudo-astrolgers into a blind panic as they tried to shoe-horn them into the traditonal Astrological system.
Here, endeth todays pompous, opionated lecture on "Why Modern Astrology is Shite!"