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 BoeSword is sufficient to defeat them.
It's a Black Boe...
Link doing battle with Black Boes.