Humans are practically chimpanzees. When female chimpanzees come into estrus, their labia swell up to enormous proportions. And since they normally walk around on their knuckles, those labia are right in front of the faces of the male chimpanzees. They form a deep, highly visible cleft that evolution primed the common ancestors of both humans and chimpanzees to take as a sign of sexual availability and desirability. Going back multiple species of ancestry over many millions of years. This sign is very common in primates.
Not very long ago in evolutionary terms, humans and chimpanzees diverged, with the lineage that led to humans adopting an upright stance. The cleft of those huge, swollen pudenda was now several feet below the gaze of the males, pointed downwards out of view, and even further hidden between the thighs. But bipedal locomotion required stronger thigh muscles than their ancestors (and chimps) had. The growth of those muscles led to a new cleft between the buttocks, far more visible than the now-hidden labia. A love of clefts went back millions and millions of years among primates because of what a cleft used to signify. And this new one was even bigger and deeper than an estrus female’s labia ever were! Male proto-humans readily adopted that cleft as a new signal of desirability. And as the females nursed their young, their swollen mammary glands created yet another cleft even closer to the males’ line of sight. The bond between the male and his female was further strengthened by his pleasure at that desirable sight, increasing the infants’ odds of survival. Over time, genes for a plump “ass” and big “tits” were selected as traits that led to offspring which survived and reproduced.
Evolution doesn’t perfect, but only hones. Male primates like clefts. It doesn’t matter that the only cleft that really does matter, the one around the vagina, was small and rarely visible. That the cleft between the mammaries and the cleft around the anus were mechanically useless to reproduction didn’t matter either. Females with clefts and males who like clefts still led to more successful reproduction and rearing of offspring. And so males continue to like clefts. The the plot of a pony is comparatively huge, right out there and visible, and (being made of basically geometric lines) is just about a perfect cleft.
It’s pure biology. That the ponies are also remarkably endearing little cartoon creatures doesn’t hurt either. But there’s no way primates with enough brain power to recognize that the ponies are intended to be creatures would miss that cleft. Or wouldn’t be fascinated by it.
