I had just been working on a project and had set the game to peaceful. About 30 seconds later, I found a small, moving, purple object emitting particles. As it happens, this matched the Endermite form. However, nothing happened when:
A. I hit it
B. I used "/kill @e[type=!Player]"
This should not occur in peaceful, and I could only get rid of it by dropping it into the void. Also, it appeared to be spinning incredibly fast and did not travel through the nether portal.