From the Super Mario Wiki
Naval Buds are young Piranha Plant enemies that first appeared in Super Mario World 2: Yoshi's Island. They grow out of Naval Piranha, which has two of them. In battle, the Naval Buds move towards the Blue Yoshi and leap strait up to hit it; Yoshi cannot destroy them, though they do not pose much of a threat.
In Tetris Attack, two Naval Buds were Naval Piranha's partners.
Naval Buds are similar to Lava Buds.