Originally Posted by brndirt1

It is true that horses can become "sea-sick", but they cannot vomit due to their physiological construction, this is because of a tight muscle valve around the esophagus. However, they develop an illness which mimics, but is NOT colic which can be fatal, it is simply a "nausea" which can be easily treated with hyoscine (which has been around since 1880) before departure or while at sea.

The biggest problem with transporting horses by sea is that they become stressed when confined to dark enclosed spaces, and can lash out at their fellow equines, their handlers, or even the sides of the ship in which they are being transported. This is why "horse killers" were often aboard ship, not to put down those who became "sea-sick", but to eliminate those who had gotten so stressed out they were a danger to themselves and others.

Originally Posted by John Dudek

All too true on all counts, but the very fact that the horses would have been penned up in closed, dark holds for nearly a month aboard ship would disqualify them from any heavy lifting duties for the invasion, because their muscles would have atrophied, they would still be drugged-up and a large number of them would still be extremely sick or already dead.