Crested Sunmare is arguably one of the most powerful cards in Hour of Devastation. A 5/5 horse than comes down and threatens to make additional indestructible 5/5 horses is nothing to scoff at. Unfortunately, the format has skewed so aggressive that a 5 mana creature that doesn't threaten to win the game on its own doesn't really fit into the format. It also doesn't help that cards like Vraska's Contempt and Cast Down don't really care about Crested Sunmare's text box. That said, if you can get Crested Sunmare going, the card is enormously powerful, and that's exactly what Cha21 has done:

Mono-White Mares | Core Set 2019 Standard | Cha21, 5-0 Standard League

• Creatures (28)
• 2 Fairgrounds Warden
• 2 Mentor of the Meek
• 4 Ajani's Pridemate
• 4 Crested Sunmare
• 4 Diamond Mare
• 4 Knight of Grace
• 4 Leonin Vanguard
• 4 Militia Bugler

• Planeswalkers (1)
• 1 Ajani, Adversary of Tyrants

• Enchantments (8)
• 4 History of Benalia
• 4 Ixalan's Binding

• Lands (23)
• 17 Plains
• 2 Scavenger Grounds
• 4 Shefet Dunes

• Sideboard (15)
• 2 Baird, Steward of Argive
• 2 Cast Out
• 2 Dusk // Dawn
• 1 Fairgrounds Warden
• 2 Forsake the Worldly
• 1 Gideon's Intervention
• 2 Remorseful Cleric
• 1 Settle the Wreckage
• 2 Sorcerous Spyglass

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At the top of your curve is Crested Sunmare. One of the strengths of this deck is that you have many ways to trigger Sunmare the turn it enters the battlefield. Both Diamond Mare and Leonin Vanguard can get that job done and let you start getting out of hand quickly.

The other exciting thing about this deck is the choice of Ixalan's Binding as the only removal spell. This card has a lot less versatility than something like Cast Out, but in exchange you get quite a bit of value. The extra text on Ixalan's Binding matters quite a bit against a number of decks. The Scarab God decks in the format frequently don't have ways to remove enchantments, so sticking a Nicol Bolas, the Ravager and a The Scarab God under Ixalan's Binding can make it very hard for them to win. You can similarly blank backup copies of cards like Glorybringer, Hazoret the Fervent, and more.

If you're looking for a budget entry point into Standard, this seems like a fine choice. You've got aggressive starts, some card selection and value, backed up by a powerful endgame engine that demands an answer urgently.