From Detective pony ch3, on horses and Void/Heart: "Anna looked him in the eye so he would know she was serious. But Acorn’s eyes were even more fathomless than usual. Looking into them was like gazing into the abyss. And the abyss did more than gaze back. It grabbed Anna by her fucking soul and dragged her down into itself." (cont)
(pt2) “ Nietzsche was a hack, it whispered to her. He thought he could even imagine what the abyss is? Identification is taming. And I, like a wild pony, cannot be tamed. I am timeless, mindless, pointless. I am abysmal, in all senses of the word. I am all senses of all words. For the sum of everything is nothing. He who fights with monsters is already a monster, for man can only fight himself.”
Not part of Homestuck canon, but definitely interesting that the topic of horses somehow led back to souls and abyss. There is a horse lurking within the bloody darkness of all hearts.