Beach Koopas are a type of Koopa Troopa. They don't have their shells on, and wear nothing more than an undershirt and/or boxer shorts. They first appeared in Super Mario World.
Different colors of Koopa Troopa react differently to being kicked out of their shells; a blue Beach Koopa kicks any shells it comes into contact with, and a yellow Beach Koopa goes into the shell, flashes colors and chases Mario.
Any Super Koopa who yields a flashing cape reverts to a Beach Koopa after being stomped on and will relinquish it's feather.
History
Super Mario World 2: Yoshi's Island
Beach Koopas are also found in Super Mario World 2: Yoshi's Island and Yoshi's Island DS, where they walk upright when evicted from their shells as an oppose to on all fours. This version of the Beach Koopa makes a small appearance in Paper Mario as the inhabitants of Koopa Village have had their shells stolen by Fuzzies.
Super Mario 64
In Super Mario 64 and Super Mario 64 DS, Mario can jump on top of a Koopa Troopa to knock it's shell out of the way, jump on to the Beach Koopa and receive a Blue Coin while the consequently dead Koopa Troopa panics and tries to get back into its shell. Mario is able to ride his shell then.
Mario is Missing!
They also make an appearance in Mario is Missing!, where Bowser can be seen in a similar form to these minions when his shell was taken off him. He was seen in boxers with heart patterns.
Super Mario 3D World
Beach Koopas also appear in Super Mario 3D World but only take this form when their shells are kicked off of them. They will chase after their shell once it's been taken, and stop and sulk when they fail to get it back after about a minute.
Unused
Beach Koopas exist unused in New Super Mario Bros. Wii they will be kicked out of their shells if they get stud on and then retreat its shell but the sunglasses and spiked collars protect them to stop themselves being kicked out of their shells and always protect them to go inside their shells. In this game Koopa Troopas and Koopa Paratroopas in green and red colors were originally going to have sunglasses and spiked collars like in Super Paper Mario but these designs of the Koopa Troopas with the sunglasses and the spiked collars were removed for unknown reasons.
Other media
In the Super Mario World TV series, the Koopa Troopa henchman of Kootie Pie Koopa is incessantly kicked out of his shell - turning into a Beach Koopa - during one of Kootie Pie's temper-tantrums.
