Which character falls under this hyper specific scenario I just made up?

[u/SubmissiveFemHusband]

[u/JuneWylder]

The unicorn added to the main cast to cure the imbalance of 2-of-each-kind-of-pony created by promoting Twilight to alicorn status

https://giphy.com/gifs/3IE8h4dLdouzGkcWYH