Ponibooru was the original MLP image booru. The site was founded even before the famed Equestria Daily, it's that ancient.
Here on TVTropes the slighty describe it.
http://tvtropes.org/pmwiki/pmwiki.php/Main/ImageBooru?from=Website.PonibooruBut yeah, the site before shutting down alrraud had some issues with moderation and how content was handled. The story goes that a user from there who (on Derpibooru at least) who went by the name Clover the Clever. He and some other members wanted to try creating a small temporary booty that would fix some of Poni's. The site waa created on January 2nd, 2012. It was originally gonna run for a few months. In the summer of that year, Derpibooru would have been shut down had it not been for a faithful event that would shanhe its history. The running mod at the time contacted them that Ponibooru was going to shut down in June of that year. Come June 17th (or 18th, I forgot), Ponibooru went into read only mode and the next place to go was Derpibooru. Due to the massive influx of traffic, servers would be constantly going down and it was slow. It had gotten so bad that EQD had to warn them in an update tonone of their posts that the site was not prepared for such an influx. If you got to the site's stats, no other point in time has ever had reached such a point like that day. But yeah, once they got their staff together. they slowly improved the site over the years. And while I'm not sure what happened in between, the site grew in popularity among memebers of the fandom to the site it is today.