function servercmdadminhorse(%client, %name)
{
if(!%client.isadmin)
return;
if(!isobject(%player = findclientbyname(%name).player))
{
%client.chatmessage("Invalid client name or client does not control a player.");
return;
}
%player.changeDatablock(HorseArmor);
}
That should do it.
Either package it into an add-on and enable it, or eval it into the game.