You know the area where you fight The Fear? Go there, either at night, or after defeting The Fear, and place a mouse trap. Walk away from it, but stay in the area. Wait for the trap to go off. If it's not a "turd snake," and is a squirrel, other snake, frog, etc. then get rid of it and reset the trap. You'll know what a "turd snake" is when you trap it. It can be found in a few other areas of the game, but not many. It's actually called a Schucenko, or something like that. It's easier to call it what it looks like. It's some sort of half salamander, half snake. It's very wide, short and brown and it flops around. You never see it unless you trap it first. Another Kojima easter egg.
Save it, never kill or eat it, and finish the game with it alive. You'll need to trap it again near the waterfall when meeting up with Eva. The reward is Infinty face paint for your next game.