A rock-boring sea urchin hitched-hiked into my tank via live rock. Started out smaller than a single 'plain m&m' candy:
A little over a year later I had to place it in my Fish Only system once it got about the size of a golf ball and started toppling over corals in my tank (including one of my trumpet corals which is nearly the size of a grapefruit, I didn't think anything could move it).
Here it is on the glass in search of coraline algae to eat:
I founds lots of critters in my live rock that I did not know of. Some only came out about a month later. Starfish, Pods, and a pinky size pistol shrimp that I have no idea how I miss. I only have 14lbs of live rock.