Metal Gear Solid 3: Snake Eater
The Kerotan first appeared in Metal Gear Solid 3: Snake Eater. Every unique area in the Virtuous Mission and Operation Snake Eater contains one Kerotan, except the Groznyj Grad Sewers. Hitting all 64 Kerotans gives the player the "Kerotan" title and unlocks the Stealth Camouflage for subsequent playthroughs of the game via a completed save file.
Hitting or shooting a Kerotan for the first time in the HD Edition will unlock the trophy "Prince Charming," referring to the fairy tale The Frog Prince. Hitting all Kerotan frogs in the same version will also unlock the trophy/achievement "It Ain't Easy Being Green," referring to a catchphrase uttered by the Muppet Kermit the Frog, to whom the Kerotan bears a passing resemblance.
Metal Gear Solid: Portable Ops and Metal Gear Online
In Metal Gear Solid: Portable Ops and Metal Gear Online, the Kerotan was an item central to the "Capture Mission" mode (essentially the same as the "Capture the Flag" mode in many mainstream team-based shooters). In the latter game, the player could also obtain Kerotan-themed Ghillie suits (top and bottom), Gas Masks, and Soft Caps if the player attended the birthday celebration for Kerotan on June 08, 10, and 12 for the two-year anniversary of Metal Gear Online.
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 the players a bonus item, such as Stealth Camouflage, Fultons, large rations, body armor, or large medical kits.
On the official website for Portable Ops Plus, the user has to locate various Kerotans to access parts of the site, 7 for each map, 14 total. They also lay hidden, requiring the user to either locate them via hearing the Kerotan's croaks, and/or using the Kerotan Locator (which fills up the closer to the Kerotan's location the aiming cursor gets). It requires at least one reload before finding all of them, either by pressing the reload button or by clicking on a Kerotan. Upon revealing them they also expose parts of the site that detail various features from Portable Ops: "New Maps," "Infinity Mission," "MPO+ Trailer," "Battle Types," "Support for Beginners," "Unique Characters," and "New Characters" on Map A (Town); "Boss Rush Mode," "AP Recruiting," "Online Trade," "New Careers," "Character Development," "Communication," and "Special Movie" on Map B (REX Hangar). Completing both maps will result in Naked Snake calling the user via radio and supplying them each time with one of two wallpapers that references Portable Ops Plus.