Let me know if I missed any! 

1) Demons
This form possibly only exists in the human world, but we're also not sure if this form is entirely made up by the dark magic. Either way, this is G4's "canon" demon.

2) Centaurs
Unfortunately the only centaur we know of is Tirek, who honestly just looks like the demon! Black and red with long, dark horns and black eyes; he's as evil-looking as evil can be!

3) Cerberus
Although not demon-looking, Cerberus is probably the closest we'll get to an MLP equivalent of Satan or Hades, as it's possibly commanded to guard Tartarus prisoners.

4) Sirens
In some cases, Sirens are also classified as Succubi, which are female devils, but instead of doing the do with men, they use their voices to entrance them.

5) Windigos
These are based off of Wendigos, which are either seen as a demon, ghoul, spirit, or other supernatural being. 

6) Rams
Goats and rams demonic biblical figures, and although we've seen sheep, goats, etc in the show, there's no ram that's as big and smart as Grogar, not to mention he has magic!

7) Minotaurs
Similar to Centaurs, except Minotaurs are sometimes considered half-human half-devils. However, these "evil" creatures are friendly enough to live amongst ponies.

8) Abada
Okay, not actually devils, but there is one Abada with the name of Dust Devil. Although, they're swift, they're friendly!