Speaking as a 33 year old dude who when he was a kid did watch the original my little pony cartoon series, since then I forgot mostly about that and grew up loving x-men, batman, computer games and all the usual stuff.
I never even pegged my interest when seeing my little pony stuff still produced years later, including when the current generation of it became popular, and so of course I wondered what this whole brony thing was based around any why it existed.
Same except I was curious what all the hubbub was about, so I checked out an episode or three, which quickly snowballed into watching the whole season and jonesing for more.
So on that end it made me wonder just why is there such a following for adult men, bronies, to get behind it. At best what I can take a stab at, is some men saw the show, and liked it, the story, the character design, the overall premise of the series, and became a fan.
Bingo! I particularly liked the characters. All six of the mane cast have their own distinct personalities, quirks, strengths, and weaknesses.
And this show has some serious talent behind it. Lauren Faust did a hell of job taking on a more or less defunct franchise and really breathing new life into it. She said she wanted to appeal to kids while still making it bearable for their parents. She definitely accomplished that. Maybe a little too well.
And John de Lancie played a nearly omnipotent being with god-like powers who reeks havoc and sows discord among the protagonists. And he also played Q on Star Trek.
And there's even Weird Al!
They told others they liked the show, and got given some negative feedback by them, which resulted in them forming a cultural following for men who felt the same, and formed a fandom around that, made even stronger when it got ridiculed by people like online.
It's the other way around, actually. There have always been neigh-sayers, even at the early stages of the fandom, but the brony fandom isn't a reaction to them, they're the reaction to the fandom.
The video gives a great explanation of the origins of bronies. Basically, 4chan (of all places) really catapulted it into the public eye. And while /b/ + pony (or alternatively, bro + pony) may have initially meant adult male fans, the term has expanded to encompass all fans of the show.
My overall thought on bronies and the haters is while I think MLP-FIM is a good show, and can understand it having a following, its always struck me as a little extreme to form a cultural fanbase around it for grown up men, not to say that its a bad thing, just that given this show came from a long line that had always been advertised for little girls, yeah, you can see why people would argue it being just for little girls and why they find it strange.
There's a lot that can be said about the little girl argument, so I'll try to be brief. But basically, the argument boils down to claiming that one shouldn't enjoy something if you aren't the primary demographic, which is just incredibly dictatorial (who are you to decide what someone else can and cannot like?) and patently untrue. Pokemon, Disney/Pixar movies, etc are all thoroughly enjoyed by people well outside of the primary demographic.
There's nothing wrong with that. In fact, broad appeal well outside of the target demographic is the sign of a wildly successful show. Okay, so that gets the age argument out of the way, what about the other half of that objection - that's it's for girls, not for guys. There's something really screwed up about that sort of sentiment - that liking something "girly" is beneath us guys, that liking a show like this is somehow a threat to our masculinity, which is a complete and udder cowpile.
Here's what Lauren Faust says about this: "I love the idea of 'all ages' entertainment - media that is enjoyable to male and female, young and adult. But the perception currently out there is that, if the main character is a girl, or if a significant portion of the cast is female, it is 'for girls' only and exclusively. And usually, if something is planned for both genders, it means most of the characters are boys - and that maybe, if you’re lucky, there are a couple of girls who get to tag along. I long for the day that female characters are not considered novel."