system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] Why is gandalf's horse such a big deal?

(Shadowfax?) It looks like any other horse yet it leaves everyone in awe when it shows up.
gpt
In the old days of the men of Rohan, more than 500 years before the days of Shadowfax, the Rohirrim lived in the North of Middle Earth near the Carrock. The ancestor of the Kings of Rohan, a man named Léod, captured a young foal and raised it into a strong and mighty horse. When he attempted to mount and ride it for the first time however he was thrown from its back and killed.

Léod's son Eorl held this against the great horse and being mighty in will and strong of hand tamed the great horse and named him Felaróf. Eorl rode Felaróf into battle when their people came to the rescue of Gondor when the Southern kingdom was in danger of being overrun by the Wainriders. As a reward for their help the Steward of Gondor, Cirion, gifted the land of Rohan to the Rohirrim and they were to forever be allies with the Kingdom of Gondor. Eorl the Young thus became the first of the Kings of Rohan and Felaróf in the new land of Rohan sired a race of horses nearly as great as himself called the Mearas. The Mearas are said to have been long-lived and of great intelligence, and Felaróf himself is said to have understood the speech of men. 

This is the ancestry from which Shadowfax emerged, and in the days of the War of the Ring not even the King of Rohan had ridden Shadowfax. Gandalf when he visited Rohan tamed Shadowfax, and from that point forward was the only being that Shadowfax would bear, and begrudgingly King Théoden granted him as a gift to Gandalf. Though they were separated shortly before the outset of the Fellowship of the Ring, when Gandalf returned to Rohan he was reunited with Shadowfax and rode him into battle both at the Hornburg and the Battle of the Pelennor Fields. It is even said that Gandalf took the mighty steed over the sea with him when he departed for the land of Valinor, allowing them to stay long united in the country of the Valar.