Names: Celestia and Luna
Species: Sun and Moon Ponymon
Type: Light/Cosmic (When fused together)
Ability: Magnetic Flare — Increases the power of
Fire-Type moves but heals the user if hit with a
Water-Type move.
Gmax Move: Gmax Eclipse — Cancels any weather effect.

PonyDex Entry:

When their powers are combined, Celestia and Luna can create a portal that
leads directly into the heavens. They can only achieve their Gigantamax form
when they work together.

Origins: Celestia and Luna are a reference to the Greek deities/siblings
Helios and Selene. Their wings forming a portal into space is also a
reference to the titan Atlas, as well as the two pillars put in place
by Hercules.

NOTE:

So I decided to go in a different direction with the sisters.
Originally I was going to give them their own individual
Gigantamax forms, but I thought it would be more fun
to combine them into one form, similarly to Twilight and
Spike. For this reason, the Gigantamax Celestia that I
had posted before was deleted.