NOTICE: Iceberg Lettuce does exist. Look it up here.
The Iceberg Lettuce in the game looks like a regular head of lettuce, except it is light blue and has icicles hanging from it.
In Mario World: Bowser's Bash
Icerberg Lettuce first appeared in this game. When Yoshi ate it, he would become Ice Yoshi, able to spit a stream of grayish blue mist from his mouth, freezing any enemies. Ice Yoshi looks like a light blue Yoshi, except his scales are silver.