From the Super Mario Wiki
A Mega Eggdozer is the term used for the massive Yoshi Eggs found in Yoshi's New Island that Yoshis can obtain by swallowing a Mega Guy and can be used to break walls or obstacles. Mega Eggdozers will replace all the eggs which the Yoshi is carrying, and will also disable its Flutter Jump. When a Mega Eggdozer comes into contact with something unbreakable, it turns yellow and then goes the other way. If this happens one more time, it turns red and finally comes off screen after another hit. They are essential in some boss battles. There is also a metal variant called Metal Eggdozer that can be obtained by swallowing a Metal Guy. These can only roll on the ground and can make Yoshi sink on water; it can also destroy Brick Walls.
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