That is the placeholder Pegasus. It uses an old, unoptimized model with textures that only look ok from a distance and were just a quick placeholder bakejob. The ship isn't released and won't be until it's ready. That basically goes for every ship that isn't in Shattered Armistice.
As for Beyond the Red Line, it was a game with the same idea behind it as Diaspora, also based on fs2_open. In fact, most of us Diaspora members used to be members of the BtRL team before a split occured, and the majority of the then-BtRL members left to work on Diaspora. As for BtRL, it only ever released a demo, there aren't any battlestars or basestars in it but it does have a couple of cool missions.
You can grab it here: http://www.gamershell.com/news_37552.html
Like Diaspora, it's standalone, you don't need fs2 to play it.