Let's blame Old School Magic and their love of old black-bordered cards... I mean, Ebony Horse isn't a terrible card by early Magic standards. This 3-mana artifact has a 2 mana ability that allows you to remove one of your attacking creatures from combat. It's a nice way to save one of your creatures from being destroyed in combat.
But why the crazy price movement on a card of which there's plenty of copies out there? The simple explanation is that the other versions are white-bordered and the black bordered version looks a lot better. The artwork is classic and gorgeous. This is an Old School Magic card if I ever saw one. And apparently, this isn't the first price spike our equine artifact friend has enjoyed. Ebony Horse hit $6 back in September '17.
Plain and simple, Ebony Horse is an awesome collectible Magic card and while it's not on the Reserved List, they don't print cards like this anymore. So if you find a good deal on one of these and you like horses, I'd grab one just out of appreciation for the Old School MTG.