Salvo the Slime is the first boss of World 1 in Super Mario World 2: Yoshi's Island
Yoshi's Island Saga
Super Mario World 2: Yoshi's Island
It began its life as a regular slime (known as a Lemon Drop) but is transformed by the magikoopa Kamek into a gigantic slime that proceeds to attack Yoshi on its master's behalf.
Yoshi can defend himself via firing eggs at Salvo, causing the slime to break into smaller parts (creating Lemon Drop slimes in the process) - eventually after hitting Salvo enough the slime will vanish and Yoshi can proceed.
Yoshi's Island DS
Though not appearing as a boss, Salvo makes a cameo along with other bosses from Super Mario World 2: Yoshi's Island in the opening of the game.
Salvo has appeared in other non-Yoshi games but still resuming to his antagonistic side.
Mario Party Saga
In Mario Party 4, Salvo returns under the name of Big Slime and has his own minigame called "Slime Time". Though he appears in four forms each of different colors such as darker yellow, pink, purple, and green variations.
In Mario Party DS, Salvo appears as the boss of the second floor of the Duel Tower. It's also revealed that Salvo can speech. Salvo will challenge the player to a Volleybomb, and if you defeat him, he will give you a mini-game as reward.