You must get the grapple before you attempt to get this. The fish statue at the pool that pulls out far from the wall has to be pulled all the way out. Then go to the left of it and pull the spear out of the wall with the grapple. Get up to where the two statues are with the spear pics on the floor. Stand looking at the statues, and go to the right and jump off the ledge and swing on the spear. When you jump off, press X. This will deploy your grapple to the ball above you. Once you swing and jump on the next spear and onto the ledge, go to the middle where the horse statue is. You'll see the green glow off it.