Grotesques were Mal'akh that had devolved, usually into the animalistic.
If unable to feed on non-Mal'akh, they would feed on each other. This would lead to their memetic field becoming unstable, and their form and minds degenerating. The term was coined by Richard Francis Burton, first appearing in the Liber Sanguisugarum. Civilised Mal'akh sometimes kept grotesques as pets. (PROSE: The Book of the War)