Re: x-rar decoder
It sounds as if you got a bunch of rar files, but it's not quite clear from your description. As for mkv, mplayer and mpv should handle that just fine.
As for why they can't settle on one format, formats change, sometimes for the better. For example, often, re-encoding a video from libx264 to libx265 can halve the size with no loss of quality. (Not always, though).
I'm not familiar with x-rar, and do wonder if just running unrar e whatever.00rar (or however it's numbered) might not do the job.
Sometimes, things are listed as free and legal to play, and then, when you extract it, there will be something like go to this site and register first. If it's something like that, I'd recommend avoiding it.