I had one of those in my last game; the sea flayers - an experiment with aboleth science upon a larval mind flayer before implantation into a human host. The end result was not unlike an aquatic mind flayer with the skin patterns of a blue-ring octopus.
I devised them as a foil for the krakidan, a human/kraken crossbreed created when a female human slave bathes in a ceremonial pool previously "prepared" by the kraken and later gives birth to the hybrid.
Did I mention that a player has a baby kraken character...sorta.... in my current game?