The Black Boe is an enemy type encountered in Majora's Mask. They are always clumped together in groups, as that is the only real way they can harm Link. They come in two varieties; the Black Boe chooses to immerse itself in dark areas whereas the White Boe hoists itself within the lightest conditions, such as snow.
Black Boes typically lurk in the dark in order to blend into the adjacent surroundings. They attack in swarms and sometimes hide in pots. A slash of the Sword is sufficient to defeat them.
It's a Black Boe...
Link doing battle with Black Boes.