doh.. I think watching Rocky IV has made me lose a few brain cells. Forgot the important part to the script.
void main()
{
object oPC=GetLastUsedBy();
HorseInstantDismount(oPC);
}
I get the same result for both OnUsed and OnClick.. Horse doesn't dismount me, it just vanishes. I am using the NWN horse, not the CEP.