This is basically the Occam's razor of web videos. Take two of the most obvious elements, pair them together, and voila, you have something genuinely pretty funny.
In this case, Ginuwine's '90s anthem "Pony" and My Little Pony dolls. We're not sure exactly of the crossover audience for the two -- are there a lot of Bronys who are into Ginuwine? -- but you can't deny the appeal of that pulsing bassline while being reminded that indeed, Friendship Is Magic.