Seriously, why are young girls so interested in horses, in a “I’m makling a drawing of a horse that I’m putting in my horse folder that I’m putting into my horse backpack before I go homa and watch a video about horses” kind of way?
They’re romantic
Handsome Princes always ride up on noble steeds to rescue Princesses
I always saw horses as a means of escape. Bold. Daring. Fast. The way to get away from my “real” life - which never quite felt like the one I should have had. They were beautiful, my brothers weren’t interested, and I knew that MY horse would love and listen only to me…
Because the boys already took dinosaurs.
I really wished my parents had bought me a triceratops to ride around on when I was a kid…
Not being a girl, I don’t know, but I do know is that after my daughter started to actually ride, and owned a horse, she lost all interest in reading Pony Club and other horsey series. She did like to read books about different types of bits and horse anatomy.
Hint for parents - buying horse books is a lot cheaper than buying a horse. 
I think that there’s an element of romance, and also the idea of having a really big pet that you can ride on and do stuff with. They’re nice-looking animals, too; very expressive looking. Also, in terms of the horse drawing/backpack/movie thing. . .kids obsess. I obsessed over Power Rangers; some people obsess over horses. So it goes.
Now, as a caveat, I never did the horse thing. I never really liked the idea of having a pet that could poop that much. And they kind-of scared me :o .
Triceratops, heck. I wanted a T-Rex. “Gee, Mr. Harrision, Rex here thinks you’re giving me a bit too much homework.”
They are hung like… well, horses.
Have you looked at ponies?
I’m a dude, but…they are so cute!
That’s why.
I think it has something to do with the idea of having control over a powerful but submissive force. In other words, it’s all kinds of Freudian and sexual.
… between their legs
Sorry, that was wrong of me. I’ve been fighting the urge to post something like that all day; but when Dio did it first, I could resist no longer.
I don’t think you were out of line at all. Masters and Johnson said many girls and women masturbate by rocking on a pillow or sofa arm. You’re certainly not the first to talk about girls riding to orgasm on a saddle.
Don’t you point and giggle, either, guys. You have those moaning pseudo-race bikes, where you lay down over the machine with a vibrating pad up against Mr. Happy. You can dry-hump the thing by shifting up through six gears and back again. Yeah, don’t you laugh at those horsey girls. 
Man, this is a huge hijack but…
When I was a kid there was a movie about some kid who finds a dinosaur egg and it hatches into a triceratops. I was walking home from school and found a big egg-shaped leather object half buried in the snow and leaves. I was thrilled, I thought it was my very own triceratops egg! Turns out it was just an old (American) football.
I was a geek, even as a kid.
Sorry for the hijack.
Yeah, I thought it was all about sex, too.
But what of those randy girls who weren’t into horses?
I can give you five reasons why that’s not necessary.
While it’s possible that young women may discover the thrill of riding I don’t think that’s why girls like them at a young age. In the US, horses are a introduced as a childhood icon to girls just like dinosaurs are to boys.
Well, I can only speak to my own experience here. While in Elementary school, I got interested in the Walter Farley books and started reading the Black Stallion series. I remember asking for a horse for Christmas one year; my parents replied, “But where would we keep a horse?” to which I responded, “In the garage!” - never having ridden a horse, mind you, and certainly not considering the reality of tending to and feeding a horse. After a year or so, I moved on to the Little House on the Prairie series.
I think it has something to do with the fact that horses are the only other animals that wear shoes. Personally, I’m lacking the gene that prompts interest in horses or shoes.
I have a half-baked theory that girls like horses in part because boys don’t like horses, so the girls are attracted to area where they are in the majority. It also helps that horses are both cute and interactive. It’s a great activity for girls – the girl is in a position of both power and partnership.
In my theory, boys don’t like horses becaues they take one look at a person sitting astride a horse and think “Oww! No thanks!”
(The boys and girls in my theory are meant to be representative of large numbers, and not reflect the interests and/or abilities of individual boys and girls)