I am glad I have a powerful PC or this could have caused a crash. I am planning on using this "bug" to my advantage (by killing them for loot) but other players (on less powerful devices) may not be able to play like this. I can only imagine how much lag this would create on a tablet or phone. 

I did NOT spawn these, they are all natural. This world has NEVER been in creative! 

 Mojang, you are going to have to fix this... Mobs do the same thing in the ocean. I have thousands of squids and dolphins around my guardian monument.