Deep one?  
Then… OMG!  
 
And if this thing exists… Then that means…  
CELESTIA HAS BEEN TRAINING TWILIGHT INTO BECOMING AN ALICORN TO DEFEAT THE ABYSSAL GOD!  

@Pomf  
There’s already the Headless Horse, remember?

@CT
 
Ah true
 
@CD
 
…There should be a Headless Horsepony villain. With a backstory and everything!

@Pomf  
Yeah, you can imagine he’s the Christopher Walken version of the Headless Horseman!

@Psychopomf  
I mean the title is boring evil unicorn :T

@CT
 
Hey. King Sombra was anything but boring.

CRAZIER FAN THEORY ALERT
 
For each species a contract with a demon changes them physically depending on how much they fulfill said contract they do grant them immense power but there are a few unmentioned…detriments to these deals:
 
a flutterpony who makes a deal becomes known as a changeling queen/king
 
a sea pony who makes a deal becomes known as a deep one
 
a pegasus who makes a deal becomes known as a windigo
 
an earth pony who makes a deal becomes known as a witch
 
And a unicorn who makes a deal well they just become an evil unicorn, rather boring if you ask me :T

Well not to toot my own horn but
 

Deep Ones?
 
I came up with that name on my own for deep sea dwelling sea ponies that look like angler fish.  
But I got laughed at.