I became a brony in c. December of 2012. At the time I didn't even know about the Brony movement, all I knew was that it was on the internet more than most other cartoons. I remember telling some of my peers that I watched My Little Pony and it wasn't until other people asked me if I was a brony when I decided to research it and join it. That's how good the show was, I developed an interest by myself, without having even heard of the Brony fandom. Within only a few months I had caught up to the then most recent episode, season 3 episode 12.
The first brony video I saw was The best of Fluttershy.