Are there rare C/B+ monsters?

Quick question from a new player here, but is there such a thing as a rare C/+ or B/+ monster? This would be mostly from a finishing your collection perspective (I assume anyway).

there are a few "rare" B/+ monsters, obtainable during events such as the current den of woe, but you really don't NEED them to get your achievements, so i really don't pay them any mind.

Thanks for the reply. I noticed that I've apparently collected enough C/+ monsters to finish that achievement with more C monsters left to be discovered.

Don't worry, you'll pick up more from the events. Events tend to unearth random monsters that you've never heard of(though that's starting to not be true for me)