Multiple dumb luck.

As Jon slept, the change began. While his upper half was spared any changes his lower half was not so lucky. First as his front changed into a girl, a number of lumps formed, two in the front, two on the side. One on the back. She shifts in her sleep, adjusting as they grew. The back one now clearly a dog's tail, The two on the side, dog heads, each quickly growing to match the size of her human top. The two in front, dog legs. with her hind legs adjusting to match as well. The final change occurs as a new dog body forms pushing the two sets of legs out. The now Cerberus taur slept peacefully until the morning.