Ohh let’s get edgyYYyyy These are of course not canon and are only my interpretation to these creatures
 
Umbrums: The Umbrums were born from the crystals that got infused with dark magic and the bad feelings the Wendigos caused to the ponies in the past. They look like a pony skeleton in the light, and a “normal” looking pony in the dark. Since they are made from darkness, the light and good emotions hurt them. They were ruled by King Odio and his daughter Princess Rabia. Rabia had a little son, Sombra, he looked like an umbrum, but everybody knew these were the eyes of a normal pony. When Prince Amadeus (seen here Royal Family Tree-NG) went to the “wasted lands” to reform the land, the umbrum showed hostility towards everyone who wasn’t like them, Amadeus tried to talk to them but being creatures of hatred the umbrums didn’t listen, both of the races wanted to protect their families from one another. Princess Rabia, knowing that her little son wasn’t safe, left him at the doors of what would became the “Crystal Empire”, erasing all his memories with her and his people. Eventually he recovered them, when he found the crystals in which his people were turned into.
 
Bat Ponies: Long ago, when the moon and the sun were still raised by unicorns, they needed ponies who could protect them even at night. They made a spell that was supposed to turn their senses into those of a bat, to stay awake at night, but instead it transformed their horns into wings, and they turned into bat ponies. The bat ponies with time ended up being in Princess Luna’s court and protecting them. They can turn other ponies into bat ponies by biting them, but they are very careful not to.