Background Pony #B1D3
Alias Celestimoon (193) <-> Mooncest (204)
We have two Nightmare Moon X Celestia tags. Celestimoon already has implications set up, however after looking through its tag history, it looks like it was made relatively recently and pushed by just a few people. Mooncest on the other hand is around seven years old if not older and just never received proper tag implications before.
Alias whichever way seems most appropriate, I would cast my vote to make Mooncest the primary tag due to seniority and a more natural tag growth.