regarding some sanity effects Edit
there is an effect in a painting in the roivas house. it turns macabre when sanity drops below a certain point.
I have never seen or heard of gigantic enemies or invincible Horrors as sanity effects. Some enemies are larger and more powerful and resilient than others of their kind, but that is a normal part of the game, not a sanity effect. Also, many of the "sanity effects" in the Twin Snakes section were carried over from the original Metal Gear Solid, so it is wrong to attribute them all to Silicon Knights. Unless something is a clear reference to Eternal Darkness, it may have just been Silicon Knights adding to the nature of the Psycho Mantis fight as well.