From Know Your Enemy Update #2

Small and hard to spot, Energyflies are new flying enemies with a twist! Unlike Batatoes (Potato Bats) and Blow Birds from the previous Know Your Enemies, Energyflies will not chase you when you are near it. Instead, these enemies tend to either stay in one area or fly around in a pattern like resembles the infinite symbol. These guys don’t look too harmful, but don’t count them out just yet. Just like Jroggers, Energyflies are also immune to your energy shield thanks to those little yellow absorbers that are attached to it’s body. Even worse for you is that these guys will often enter it’s spiked state, where they change color completely and grow mini spikes on its body.

The spikes won’t instakill you like normal purple spikes would thanks to its shape and size, but in this state, Energyflies are both immune to your bounce attack and energy shield, making them very hazardous in the air.