From the Super Mario Wiki
Ads keep the MarioWiki independent and free :)
Flounderflages are red or green flounders with yellow underbellies that appear in Mario & Luigi: Dream Team. In the field, Flounderflages camouflage themselves into the sand, giving chase to Mario and Luigi upon sight, and becoming more visible once doing so.
In battle, Flounderflages begin by camouflaging into the sand, thus rendering them harder to see. They can attack by sliding steadily into a bro without pausing, which can be jumped over. They also may stop before the bro they are targeting to compress themselves to shoot themselves forward, first pointing either at the bro or at an angle that would over-shoot over the bro, and flinging itself in that direction. If the Flounderflage points at the bro, it must be jumped over. If not and it aims over the bro, they can simply idle to avoid it. The color on the back of the Flounderflages warns the player which bro it will attack when it does; red for Mario, green for Luigi. Additionally, Flounderflages may spawn out of the sand over the course of battle. Navy Flounderflages are rarer and are only in the Torkscrew battle when Mario or Luigi are being chased by Torkscrew's drill.
Names in other languages