Super Mario World
Super Mario World is one of the few games where Yellow Shells are actually worn by Koopa Troopas. Upon jumping on a yellow-shelled Koopa Troopa, it loses its shell and a coin pops out from it. Standalone yellow shells also appear in some levels. Like other Koopa Troopa shells, the Yellow Shell can be launched when the Koopa Troopa wearing it is jumped on and removed from its shell. It can also be held and thrown. If a Beach Koopa goes inside of one, it becomes a flashing shell and chases after the player. When Yoshi lands with a Yellow Shell in his mouth, he can create sand clouds on either side of himself while he has the shell in his mouth or before he is about to swallow it, which defeats nearby grounded enemies. (Yellow Yoshi can use this ability from any shell color.) Yoshis can also spit the shell back out, in which case it will behave like a Green Shell, flying out at high speed and knocking over all enemies in its way. If the player attempts to launch the shell back at a blue Beach Koopa, it will grab and kick it back at the player.
After the player completes the Special Zone, or find all exits in Super Mario World: Super Mario Advance 2, like with all Koopa Troopas and Koopa Paratroopas, the yellow variants become Mask Koopas, with their respective shells becoming yellow-capped masks that vaguely resemble Mario.
Mario's Time Machine
Yellow Shells also appear in Mario's Time Machine for the NES, replacing the common green-shelled Koopa Troopas. Unlike in Super Mario World, jumping on one does not release the Koopa out of its shell and instead causes it to retreat to their shells, and they can only be kicked to launch, knocking over all enemies in its way. Bodyslam Koopas also wear Yellow Shells, but jumping on them instantly defeats them.
Getting hit by the shell simply stuns Mario temporarily, rather than damaging him.
Paper Mario series
Paper Mario: The Thousand Year Door
In the Glitz Pit (Chapter 3) of Paper Mario: The Thousand-Year Door, the KP Koopas, which consists of a KP Paratroopa and two KP Koopas, including the team's leader, KP Pete, use their Yellow Shells to attack Mario and his partners during battle. They are simply Koopa Troopas with their shells dyed yellow (gold in the French and Italian releases).
Mario Party Advance
Yellow Shells appear in the Mario Party Advance minigame Shell Stack. They appear after a few Green Shells have been launched; once the Pipe Cannon turns yellow, it shoots out a Yellow Shell, which is slightly faster than the Green Shell.
Puzzle & Dragons: Super Mario Bros. Edition
Dr. Mario World