They are ponies. What sane man/boy would watch a kids show that centers around talking ponies? It's like if Barbie has a drama show that's similar to Housewives. Anything like Supergirl is different since it's a world of both genders and the main character is a heroine, which doesn't determine anything.
I believe I believe that was referencing the show name, as it draws attention to a female audience (suggested via the name), only assuming though... Much like how He-Man is suggested to be for a more male audiance, and She-Ra for the opposite (back in the day)
Ponies are gender neutral compared to one of the two genders... There are both males and female horses...
Other than that, I can't think of what made society think horses were this feminized creature... I mean, the saying
"This car has that much HORSE POWER" for a reason... I'd think.