@teslakana  
This may be a bit outside the box, but how about Arimaspi as a Black Lantern?
 
“As Blackest Night enshrouds us all,  
The cold Abyss begins to call.  
We rise again to see you fall  
And feast upon the hearts of all!”

@teslakana  

 
Celestia and Luna?  
Or maybe Black would be Tirek?

Red:  
With Blood and Rage of Crimson red.  
Ripped from a corpse so freshly dead.  
Together with our hellish hate.  
We’ll burn you all… THAT IS YOUR FATE!
 
Orange:  
What’s mine is mine and mine and mine.  
And mine and mine and mine!  
Not yours!
 
Yellow:  
In blackest day, in brightest night,  
Beware your fears made into light.  
Let those who try to stop what’s right  
Burn like my power Sinestro’s/Arkillo’s might!
 
Green:  
In brightest day, in blackest night,  
No evil shall escape my sight.  
Let those who worship evil’s might  
Beware my power–Green Lantern’s light!
 
Blue:  
In fearful day, in raging night,  
With strong hearts full, our souls ignite.  
When all seems lost in the War of Light,  
Look to the stars, for hope burns bright!
 
Indigo:  
Tor lorek san, bor nakka mur,  
Natromo faan tornek wot ur.  
Ter Lantern ker lo Abin Sur,  
Taan lek lek nok–Formorrow Sur!
 
Pink:  
For hearts long lost and full of fright,  
For those alone in Blackest Night.  
Accept our ring and join our fight,  
Love conquers all with violet light!

@darknessRising24  
The Black Lantern Oath is a great fit for Flutterbat.  
The Blackest Night falls from the skies,  
The darkness grows as all light dies,  
We crave your hearts and your demise,  
By my black hand, the dead shall rise!

Think Twilight is a bit more suited to Indigo, Compassion is essentially Friendship. And Fluttershy more suited to a Yellow lantern. Her stare puts fear into everyone.  
Spike is super fitting for orange.  
Everyone else is semi-fitting, but work with what ya got.
 
 
@teslakana  
Well we’re out of (fitting) main characters, but they’re also the rarest appearing Lanterns, so I’d say Celestia for White and Chrysalis for Black, for sure.

@teslakana  
Well at the moment in my story, the main Black Lantern and Villian bis Nightmare Moon, the White Lantern has yet to be revealed.
 
 
@FLUTTERBADASS  
Why?

Fluttershy would make a much better Black Lantern.

I wonder who the white and black lanterns would be…

Yes, I understand, but a bit more fun Yes it’s also all colors of the rainbow!

@Background Pony #AFD9  
Well technically she’s an indigo.

I love Fluttershy and blue color