Catherine actually died of a stroke in bed (albeit the stroke afflicted her whilst straining on the toilet, rather like Elvis).
So what is the story behind the horsecock-related fiction itself?
Snopes suggests that it was the work of critics who wished to undermine Catherine's prestige and reputation, suggesting that to have a strong female ruler was considered unnatural. Others blame the French (as is usual). Still others point to the fact that whilst Catherine didn't have sex with animals, she was certainly well-known for being "hot to trot" with well over two hundred men.
The wider point here, relevant to feminist analysis, is the way in which sexual innuendo has always been used to undermine powerful women. Marie Antoinette was also accused of zoophilia within her lifetime, and whilst beastiality is rarely mentioned today, sexual innuendo is still used as a weapon in the armoury of the patriarchy against women in the public eye. An interesting footnote to this is that in Nancy Friday's books interviewing women about their sex lives zoophilia is surprisingly common and there are a number of allegedly true stories of intercourse with dogs. But not horses.
And with that Kuronia, I leave you, but see, diaries can contain both horsecock and rendeeming historical and sociological content.