Kerotans

Kerotans are dolls of cartoon frogs that are hidden in various areas in several games. Kerotans are easily recognized by their distinctive croaking sound. The kerotan first appeared in Metal Gear Solid 3: Snake Eater, and has since appeared in Metal Gear Solid: Portable Ops, Metal Gear Solid: Portable Ops Plus and Metal Gear Online.

Metal Gear Solid 3: Snake Eater
In Metal Gear Solid 3: Snake Eater, every unique area in the Virtuous Mission and Operation Snake Eater contains one kerotan. Hitting all 64 kerotans gives you the "Kerotan" ranking and unlocks the Stealth Camouflage for future replays of the game from a completed save file.

Metal Gear Solid: Portable Ops and Metal Gear Online
In Metal Gear Solid: Portable Ops and Metal Gear Online, the kerotan is an item central to the "Capture Mission" mode (essentially the same as the "Capture the Flag" mode in many mainstream team-based shooters).

Metal Gear Solid: Portable Ops Plus
In the "Infinity Mission" of Metal Gear Solid: Portable Ops Plus, kerotans are hidden objects in various stages. Finding and hitting a kerotan gives you bonus items.