in the question "What can I breed a Heatran with?" the answer says " undiscovered egg groups cannot be breeded so no expections" I thought they can breed...... can we get a Kyurum if we breed Reshirom and Zekrom.... I am curious to know.
A Pokémon in the Undiscovered Egg Group cannot ever breed with anything. Simple as that.
Source: Knowledge, I wasn't foolish enough to try it from experience ;)