Pandemonium is a Draconequus that was roaming Equestria during the war.
During the War
Pandemonium was captured by the Ministry of Peace in the hopes of using his chaotic abilities for healing purposes. Pandemonium himself proved far too powerful to be directly controlled and given instructions, so they broke up his form and his magic and sealed it within four specially designed devices called Black Cubes. The Black cubes were sent to various Ministry of Peace research facilities for further study, two of which ended up in Orchard and the Ministry of Peace's Hornsmith, regional headquarters. One was also at the Stadium but relocated.
After the war
Pandemonium remains contained within the Black Cubes. His presence unsettles and frightens the old Ripple personality inside Ripple's head. Pandemonium managed to speak with Ripple as he was trying to place the second Black Cube in its special case. Pandemonium states that all he desires is freedom so he can leave Equestria. Apparently mostly everyone he could desire revenge on has long since died. He is cut off during his conversation when Shade places the cube in the case for Ripple who had been unmoving for several minutes, brought into his own mind by Pandemonium.
Pandemonium converses with Ripple again when he is nearly killed by Skyline and Cinder Trails. The Draconequus occasionally speaks with Ripple on other occasions before all four Black Cubes are stolen and brought to the Maremack facility by Hate. Pandemonium's powers are used by Epiphany to create abominations to help protect the Maremack facility.
Pandemonium is revealed to have made a deal with both Ripple and Hate. He decides to stay in Equestria, contray to what he told Ripple and heals Ripple's injuries before possessing Hate's body. Pandemonium uses Hate's body to kill Epiphany for using his power and then contemplates where he might locate a more permanent body as Hate's will degrade due to his presence. Ripple catches Pandemonium unaware and manages to destroy his vessel i.e, Hate's body, causing both himself and Ripple to vanish.
Like other Draconequus, Pandemonium is a mish mash of different creature parts.
Pandemonium st first, appears to be simply desperate to leave Equestria. Though it was revealed that this was only part of him. The rest of his essence, contained within other Black Cubes, desired to inflict untold destruction and vengeance on all pony kind. He is a manipulative trickster, folling Hate so he could steal his body and planned to try and steal the weakened Discord's body.
Pandemonium possesses incredible, godlike powers. His powers have been shown as capable of creating monstrous abominations out of nothing. Pandemonium's power when harnassed by the Black Cubes, could also bestow fast regeneration on mortals, like with the Endless ghouls. Another Black Cube could also cause accelerated plant growth.