There are nine of them. The come single if you don't pull them past another closer than they already are.
They AE rampage. Hit max for 784. Seems to have more HPs than normal mobs, but not sure.
Be sure to shrink everyone in group and make sure you're fully submerged and on bottom of the water in order to be able to cast on it. At full size, no one was able to cast on it due to the code that prevents spells going from above water to a target in water.