Chimera

Chimerism in Axolotls is said to be when two eggs fuse together in development, and each side grows according to the egg it came from, often resulting in a split-down-the-middle appearance. One side can often grow at a slightly slower rate than the other.

Chimera cannot be duplicated in breeding, it is caused by an accident during development. The chances of an Axolotl being a chimera and surviving is a 0.00001% chance. There is also the argument that chimerism in Axolotls don't actually exist, but are actually mosaic and the color variation just happens to be split.