When Beastmen were first fleshed out in "Realm of Chaos: The Lost and the Damned", Ungors were stated to be the catch-all term for any beastman that had horns, but which didn't recognizably take after a goat or a bull in their appearance. Whilst considered somewhat laughable, especially by lower-ranked beastmen, ungors were still recognized as fully-fledged gors and treated with the respect they deserved - especially those lucky enough to have one or more particularly finely developed horns. An ungor could be anything from an antlered stag-man or moose-man to a humanoid unicorn to cursed with small stubby horns, but regardless of what their horns looked like, they were still respected as fully fledged gors - unlike those beastmen who completely lacked horns, which were known as Bray. Ungors and sometimes even a particularly blessed bray could rise to become powerful Chaos Champions and herd-leaders.
In 5th edition's "Realm of Chaos" army book, ungors and bray were rolled together as the more numerous and weaker substrain of beastmen, with a tendency to have very unimpressive horns if they had any at all; in this new lore, "ungor" was the official name for these runts, but they were sometimes also known as "brays". In this edition, ungors and gors fought in mixed units.
6th edition and 7th edition doubled down on this; ungors are now the lowliest dregs of beastman society, mocked and outcast for not having horns like those of a proper gor. Bitter and resentful, they burn with the urge to take out their pain on all other races. Ungors became their own distinct unity in 6th edition, and were essentially the beastman version of Chaos Marauders. 7th edition's "Beastmen" army book added the Ungor Raiders, which are ungors with the Skirmishers ability.