What The Heck Is This?

Looks overpowered as **** dude. Simple things can make things overpowered. Plus, it looked like it was an act of X-Ray, as you saw the Skeleton through Solid material.
That's the massivemob. It has a glow effect similar to premiums /glow. That's not part of the mod I'm using. 5zig is an armor HUD mod with a few other features like a coordinate HUD.
 
Hmm.. it might be a new kind of massive mob, probably a boss, depending on how strong it's attacks are. The reason why it's probably not listed on the website is because it probably just got released recently. Or.. probably just a extremely rare Easter egg , i mean who knows..

All i can say is that must have been a lucky find.