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. It can be killed in Link's Awakening using the Magic Powder or Boomerang, and in the Oracle Series using the boomerang.

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

Quote from 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."

It should be noted that using the Boomerang on Sparks will turn them into fairies.

Giant Spark
In Link's Awakening there is a giant spark that is found only once in the game within Face Shrine. The enemy cannot be harmed with any of Link's weapons. The only attacks of the Giant Spark, is if it simply makes contact with Link, in which it will take cause 1/2 a heart of damage.