They come in lots of different shapes and sizes. Usually witches and horsemen in a skeleton form,the others are skeletons, satires, ghosts, harpies and small imps. The horsemen have strange horses with bovine heads.
Chernabog's minions appear to be quite evil, cunning, dark, and mischievous for their personality.
During the "Night on Bald Mountain" sequence from Fantasia, Chernabog summons numerous ghosts, demons and monsters from the graveyard of the village which Bald Mountain overlooks. They are supposedly to represent the pagan religions interpreting them as evil. Being summoned by him, they transformed into animals for Chernabog's delight as he throws them into the fire. With his own powers, he reveals that they are as disposable as they look, burning them in his own hellish flames. They, like Chernabog, are vanquished when the morning arrives.
The scene from "Night on Bald Mountain" is represented as Chernabog sends his army of demons to ruin Mickey Mouse's imagination under the orders of the Evil Queen. We see the face of a skull, a witch, and many other phantoms. They were destroyed in the final battle between Maleficent and Mickey Mouse, restoring the powers of Light.
One of the phantoms riding a demonic horse makes a cameo appearance as the Horned King in one of Hen Wen's visions. Other of the minions, famous for switch his color white to black two times and return to white is very similar to Horned King in a smaller size, Chernabog and his minions must have been a reference for theanimation of Horned King.
The demons appear in the two Night on Bald Mountain-themed transition levels.
All the "species" of minions appear to represent something considered bad or sin in the Christian religion. The witches and demonic horsemen are a allusion to the holy inquisition. The harpies and satyrs represent the paganism, the skeletons can be a metaphor to the "dark age" because skeletons were used by the scientists of the time for study the nature and the human anatomy. Chernabog represents Satan running the bald mountain (hell) with his imp minions and punishing the ghosts for their sins (as is believed in the Christian religion).
In "Fantasmic!" the songs have been remastered (not just Chernabog's theme but "Pink Elephants on Parade","Poor Unfortunate Souls" and the instrumental played when Evil Queen transformates) and the minions earned fantasmagoric sounds like screams and cries.
The ghosts come from their graves in a cemetery, but some comes from gibbets. Leaving implicit that they disobeyed a law in life.
Some of Chernabog's minions (the witches, ghosts, skeletons and henchmen) return to their graves and reform when the light refuses Chernabog, but they possibly were called in for another Walpurgisnacht.
In Once Upon a Time, Chernabog's minions don't appear. However the Wraith had a similar appearence to the ghostly minions with red eyes and cloak.