A Baalgar demon is a type of large "shadow lord" in the hiearchy of Hell.
Background
Doom 2016
The petrified remains of a Baalgar demon were brought back from Hell during one of the Lazarus Hell tethering expeditions (2143) and transformed by the UAC into the formidable "Cyberdemon," through a mixture of the Baalgar demon's remains and UAC technology. The implanted technology replaces the parts of its body that were destroyed during or after its death, most notably a weapons system entirely replacing its left arm capable of discharging both conventional explosives and the same energy-based attacks utilized by the gun-arms of Possessed Soldiers. The UAC technology is powered and maintained by an Argent Accumulator. A poor choice in design gave the Cyberdemon the ability to activate itself, and after rampaging through the Lazarus Labs installation, the Cyberdemon confronted, and was subsequently destroyed by, the Doom Slayer.
Doom Eternal
A smaller Baalgar demon skull can be seen inside the Cultist Base, frozen into a wall of one of the shafts, and held up by chains. This one doesn't appear to have any evidence of cybernetic modifications. The Tyrants, which take the role of the Cyberdemon in this game, notably don't look anything like the 2016 version of the creature, and thus are probably not Baalgar demons.