Umbrum (also known as Shadow Ponies) are the supporting antagonists of the Siege of the Crystal Empire arc of the My Little Pony: Friendship is Magic IDW comic series. They are an ancient race of wraith-like beings who used to terrorize the Crystal Empire before getting sealed away. They transformed one of their members into a pony to escape their prison, whom later became King Sombra. Sombra's childhood friend Radiant Hope later discovered the Umbrum, being fooled into thinking they were innocent creatures who had been wrongly imprisoned, and starts living with them, determined to some day free them.

Umbrum are large, smokey beings resembling horse skeletons, with small butterfly wings, pale colors, and human-like teeth. They have pure white eyes, which often have smoke flowing from them. In their disguised forms, they are much smaller and less threatening, with small, dark gray pupils.

https://villains.fandom.com/wiki/Umbrum