I am following the anime through EpisodeCalendar and there is no episode 12.5. Is this episode a filler or part of the original story? (That is, would I miss any part of the story if I skipped this episode?)
Durarara episode 12.5, "Justice Is Slow But Sure" isn't a recap episode. It and episode 25 are DVD only episodes that actually have some semblence of plot to them (as much plot as you get with Durarara). It's not related to any of the main plotlines, though, especially the:
overarching yellow scarves/dollars that's part of the later half of the first season.
So it's kind of a side story. You can treat them as OVAs.