I'm reading the book for the first time as well. Wrapping up the novel-only scene with Johnny Fontane if I recall correctly.

Anyways, just had a thought here: why put the head in the bed? If Woltz was so sex-driven, as we saw with not only the starlett, but also his comment on the horse's endowment, why not put something else in the bed? Woltz wanted to breed Khartoum, and losing that would certainly make life hard for him and the horse. Perhaps the Don just wanted to be nice to the horse, or maybe there was a thought that the head would be more recognizable, as opposed to, "What in the world is that?"