It looks like Princess Celestia is getting her own card in Hour of Devastation! Well, ok, not really, but it's as close as we're going to get. I mean, it has 'Sunmare' in the name, and protects all your other horses.

6003704
Mythic Rare mono-white Horse that contains the word 'indestructible'?

Never thought we'd get so close. Just slap flying on there, maybe give her her own indestructibility for extra utility and flavor, and boom! Perfect Celestia.

6003704
There's a story here waiting to happen. A sunmare desperately trying to protect her herd as the Hours tick past, only for her Spark to ignite...

Really, why should Samut get all the fun?

And I just know I'm never going to open that Horse token.

6003704
Horses exist in all 5 colors. Time for a 5C legendary horse to build a Horse tribal EDH deck~!

White:
Armored Warhorse
Crested Sunmare
Hipparion
Trenching Steed
Wispmare (thematic)

Blue:
Aquus Steed (thematic)
Breaching Hippocamp
Noble Steeds (thematic)
Phantasmal Mount

Black:
Black Carriage
Fleshmad Steed
Ghoulsteed
Nightmare
Stallion of Ashmouth

Red:
Dwarven Pony
Thundermare

Green:
Reins of the Vinesteed (thematic)
Sacred Prey
Tarpan
Temur Charger
Timbermare
Zodiac Horse

Multicolor:
Carnival Hellsteed
Spitemare (thematic)

Colorless:
Akroan Horse
Bronze Horse
Chrome Steed
Clockwork Steed
Ebony Horse (thematic)
Iron League Steed (thematic)
Workhorse

Also, cards that have/grant/care about horsemanship, pegasus cards, centaur cards, and Horseshoe Crab, because why not?

6003720
Crested Sunmare is white, not blue.

6003756
That was a typo