Prior to the Monsoon Boss fight, whilewatching the scene, the perspective of the player is in first person view. If you look to your right, eventually the PMC soldier will begin playing with a cat. Also, at the beginning of mission R-01, you can find a cat on the beach. When you try slicing it, the cat will do backflips and avoid all your attacks.
While exploring a sewer, Raiden will come across an enemy with a small boy. During the intermission sequence, the boy will shout "A ninja?! Cowabunga!", then say "Go ninja, go ninja, go" to Raiden, a reference to Vanilla Ice's "Ninja Rap" song from Teenage Mutant Ninja Turtles 2: The Secret of the Ooze.