Before you fight Nero the Sable in the Chapter 10, keep an eye out for some barrels above you as you run on through. There will be four barrels that blow up in a line. One of those barrels contains one of Final Fantasy's best known hidden character "Cactuar". As it falls down to the ground it will start running away and as soon as it gets to the corner it hops on up and disappears.

A second appearance occurs on the WRO Causeway. At the top of the right hand tower from where you start the scene, look to the very top of the antenna. You'll see Cactuar at the top, doing that weird little Cactuar dance. Not sure what shooting him does, but there ya go!

A third appearance in Chapter Ten (Shinra's Dark Secret). Towards the end after you get engulfed by Nero's Shadow, follow the staircase upwards a few flights. You will then come to a landing where two laser fences will block off your path forward and back.

Walk forward some to fight four DG soldiers and a rigged vending machine. The next room is divided into four miniature rooms. The first is a causeway with several mines. Go through and arrive at a causeway with four hovermines and several boxes. The next place is a huge gaping hole where explosive barrels are being hurled through. Navigate this and come to a room with a broken causeway. A timer will appear.

Ready your rifle and zoom in across the hall. The Cactuar is small, so center your scope in the middle of the adjacent door, at the bottom. When the timer reaches 0, a cactuar will make a two second appearance, and you have about that amount of time to shoot him.