Spark

The Spark usually appears as a flashing ball which clings to groups of pots or blocks. It first appeared in A Link to the Past as a ball of flashing yellow/blue/red light which could not be harmed. Can be killed in Link's Awakening using the Magic Powder, and in the Oracle Series using the boomerang.

In Phantom Hourglass, they are invincible, and are always found in groups.

Quote from The Legend of Zelda: The Minish Cap Spark figurine:

"Appears in dungeons. They cling to walls and move quickly. Normal attacks may not work, but the boomerang is pretty effective."