The Antique Bronze Lion is the eighty-first treasure that can be found in Uncharted 2: Among Thieves.
The Antique Bronze Lion is found in Chapter 22 - The Monastery, in the narrow room you enter just after climbing along a beam. Acquiring this treasure is easy, though you should wait until this location is completely devoid of soldiers before attempting to go after it. At the far end of the room, you'll need to work your way leftward through a door to end up back outside. Before you do that, look upward along the far wall, and you'll see the shimmer of a treasure hanging above the window. You'll need to shoot it down, but to actually get up to where it falls, use the broken pillar to hoist yourself up high enough where you can leap over to the window sill.