The Pony of Shadows is the name given to an Umbra that had taken over Stygian, appearing as one of the antagonists of the Season 7 two-parter.
The Pony of Shadows was a very large, Alicorn-like being. His body was completely black like shadows, his mane and tail gray and resembling fire. His wings, when summoned, resembled skeletal bird wings.
The Pony of Shadows was shown as malicious, easily willing to kill his opponents without much hesitation. He is shown to be very set on revenge towards Starswirl the Bearded and the other "Pillars of Equestria", though it is unclear if this was his own, or if it was influence from Stygian's bitterness towards them.
Over a thousand years ago, the Pony of Shadows became one with Stygian once the unicorn found him, the Umbra telling Stygian of a power beyond anything he could imagine. Shortly after finding the Pillars though, he and them were banished off to Limbo.
The Pony of Shadows was able to return to Equestria thanks to Twilight Sparkle releasing him from Limbo alongside the pillars. He mocked Starswirl for his failed attempt to keep him imprisoned, and "thanked" Twilight for assisting him just before he destroyed Ponhenge, making Starswirl's previous spell useless against him. He tried attacking them, but was quickly stopped by Starlight Glimmer and Twilight. Seeing the odds against him, he vowed that the realms will turn to darkness, and they all will bow to him, before disappearing.
As he searched Equestria, he quickly found the realm was far too bright for him to regain his energy, and quickly found a more suitable spot within the Prime Kindergarten. Once the Crystal Gems, and Elements of Harmony arrived in the Kindergarten, he confronted them alongside his Windigo forces. When accused of trying to steal their magic, he questioned if Starswirl was the one who told them, calling them all selfish, and stating he will destroy everything they've created. He attempted to shatter the Elements of Harmony, but was stopped when ambushed by Steven Universe and Connie Maheswaran. When the Windigo brought Steven to him, he didn't hesitate to crush him underneath his hooves, presumably killing him, and almost shattering his stone in the process. As the fight between Changelings, Tindalos, Gems, and Elements raged, he stayed aside and watched them all fight eachother rather than fight himself. Eventually, upon learning the truth about Stygian and given an apology, the Pony of shadows refused, and attacked them all, poofing all the Gems present and trapping the Ponies, forcing the Changelings and Tindalos away. He was ready to finish them all off, when Ruby came in an ambush, hitting him back. With the combine efforts of the Pillars and Elements of Harmony, Stygian and the Pony of Shadows were separated, and the Umbra was encased in the Fire of Friendship and Elements of Harmony, disappearing.
The Pony of Shadows has numerous abilities standard for Umbra; including capable of turning into smoke and shadows at will, and able to draw power from the darkness surrounding him.
While unable to do in his weakened state, he was shown to be able to summon Windigo to his aid once he grew powerful enough.