I too know a kitty who uses the toilet. He trained himself. She ran into a problem though, when he had to have a prolonged stay in the hospital as he did not want to use the litter box.
So for anyone with a cat who uses the toilet, you should keep a littler box available too.
Me, I like to monitor poop and pee to make sure all is well with them. A cat using the toilet, unless you kept some sort of bowl above the water to catch, would cause me too much stress.