Different MC versions. Bouncycastle occurs on vanilla MC as I've had it happen to me and several friends. It IS a Mojang error.
Figured it out on my own, so I'll post my solution, in case it helps anyone else.
Turns out the launcher failed to download all of the required libraries during the first installation of the Horizons' ModPack. Given the way the installation tends to go (Installs multiple files without much indication of this, which could be seen as freezing or the client failing to run), it seems highly probable that interruption of the installation process occurred, somehow. After making note of the number of files and their sizes in the installation directory, I changed the directory to another folder and tried again, and surely enough, saw more files downloaded this time around, which then worked and produced the expected, correct experience. The libraries that were missing appeared to be related to the specific MC version the modpack was running on, as well and as you mentioned, as the other 1.6.4 version modpacks also failed to work prior to this, but now work correctly.
So, to sum it up, fixing this required changing the directory in which the modpacks are installed, which is done via the Options tab in the Launcher. I suppose you could also delete the Libraries folder in your FTB installation directory to save some time, as well, but fresh installs are a good habit to have.
Hope that helps anyone who was having the same issues as me. Good games, everyone.