[b]Mars[/b]
Once the DNA was analyzed it was found that there DNA was corrupted. The Genes were all in the wrong places, and were undergoing rapid decomposition, event the best geneticist in the world with all the best equipment wouldn't be able to reconstruct the Equestrians. They also died very quickly due to plague and the abuse they were shown.

((Explanation: The plague spreads through magical links, which there suits had with a ship which in turn was connected to planets. While its normally only affects magical races,due to the nature of the plague its affects Equestrians even when there non-magical. This is also part of the reason why it didn't spread to the humans, they lost there magic fast enough that it couldn't start spreading.))