The Goner is a ghostly figure that haunts the forests and caves of your Minecraft world.
When it spawns, it stands motionless in place. When you're in the dark areas the Goner spawns in, its light captures your attention. If you don't look at it, nothing happens. But if you do, it will turn off its light and attack.
The Goner has only 10 hearts of health and deals only 2.5 hearts of damage, but it makes up for this with its incredible speed and teleportation abilities. It attacks in a similar way to an Enderman, but moving even faster.
It will randomly flash its light on and off as it chases you, and you can only deal damage when the light is on.
When killed, it drops 0-2 Phantom Membrane, 0-2 glowstone dust, 0-1 blue wool or a mix of the three.
Alternatively, you can get a lantern and 2 blue wool by cutting the Goner's cloak with shears, killing it instantly.
I think this mob would be beneficial to the game as another way of getting Phantom Membrane, and it add more variety to the mobs you fight since its another ghost (the only other one I can think of is the Vex).
Here's some concept pictures of the Goner's design and how it would appear in game (sorry for the low resolution, I need to shrink the picture for it to be small enough.)