Grass has a single resistance to electric and Solar Beam deals massive neutral damage. Not as good as ground with double resistance and super effective damage, but still okay.
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Ground types (like Rhydon) have relatively terrible moves and only decent attack at best. As such, you'll see high-dps neutral counters, such as Mewtwo/Dragonite/Machamp/Exeggcutor, in the rankings. Resisting Electric attacks is a plus.