It may be a monster, but it's a rather advanced example, no? I'm not sure why its face has a Salvador Dail tattoo of a cow running on the plains, but that just may be the way of its kind. Review:

The huge hydrogen-breathing Partussians (Nuls) have controlled Earth (one of millions of worlds in their vast empire - surprising?) for thousands of years. All of Earth's cities are long abandoned and the standard sort of Partussian city-with a hydrogen atmosphere-is built on each world as its capital (this concept is interesting, sadly, the cities are never really described). The Partussians exploit Earth for its natural resources.

Another review says Aldiss based it on the British occupation of India, and you can see his point. The depopulated ruins of Calcutta and Bombay certainly are poignant sights.

The cover of the re-release shows big Pratussians with three eyes on stalks and glass helmets, but in a different style.