I think I might be the most recent one to join the herd. Here's my story.
Sometime in February of 2016, while lurking on YouTube, I saw a video called "Animated Atrocities," by a YouTuber called TheMysteriousMrEnter, pop up in the recommended videos pane.
Out of curiosity, I clicked on it and watched. I liked the video series enough to continue watching more and more episodes, and along the way I noticed how Mr. Enter was blatant about himself being a brony.
I wondered to myself, "what is so great about MLP that caused such a huge fanbase?"
By the time I started wondering this, it was already midway through March. I had read some basic background information about the show from various websites, and I decided "hey, maybe I should give it a shot."
So, on March 28th, I sat down at my computer, pulled up KissCartoon, found MLP, and started off fairly enough at the very beginning, watching it with an open mind. After watching a few episodes, I found the show to be plenty good enough to warrant such a large and devoted legion of fans.
"And that's how Equestria was made!"