From Zelda Legends Wiki
A skull surrounded by fire. It can be turned into a fairy.
| Appearances: ALttP SNES, LA, LA DX, OoS, OoA, ALttP GBA|
Attack Pattern: Floats around, bouncing off walls. If you touch one you lose health.
|| Appearance: A Link to the Past (SNES & GBA)|
Quick Facts: Anti-Fairies appear in several dungeons in ALttP. When you touch them you'll lose health and some MP. If you sprinkle it with Magic Powder it will turn into a Fairy.
|| Appearance: Link's Awakening (DX)|
Quick Facts: In this game Anti-Fairy can be turned into a Fairy also with the Boomerang.
|| Appearance: Oracle of Ages / Seasons|
Quick Facts: These enemies are exactly the same as in LA.
Anti-Fairies look like floating skulls with four little balls surrounding them. The creatures are not very common in most games but can be found in some dungeons.
A Link to the Past
Anti-Fairies are in located in a few dungeons like Eastern Palace and Palace of Darkness for example. Anti-Fairy will always move diagonally and when it bumps against a wall it simply bounces back in another direction, moving aiminglessly through the room. These guys are immume to all attacks except magic. Sprinkle some Magic Powder over them and they turn into a Fairy. This little trick can be quite useful if you are low on hearts.
Link's Awakening (DX) & Oracle of Ages / Seasons
In these games the Anti-Fairy is identical in every way (apart from the monochrome color in LA). In LA you can sprinkle them with Magic Powder (just like ALttP) in order to turn these into a little Fairy. Since you don't have magic powder in the Oracle Series you can't take them out this way. In both games you can use the Boomerang to turn the Anti-Fairy into a fairy.
The name Anti-Fairy is fitting since this enemy will turn into a friendly Fairy when you hit them with Magic Powder or a Boomerang. Anti-Fairies in ALttP have red bells, the other games all feature a Yellow/Black Version.
When you are locked in rooms which can only be opened once all enemies are defeated, Anti-Fairies don't need to be killed. If an Anti-Fairy makes contact with you in ALttP you not only lose health, but also some Magic Points.