I think it’s a mule that’s an infertile cross between a donkey and a horse.

But we need to also consider that horses were significantly smaller back in those days. People will often say a pony is a smaller breed of horse, but it’s actually more accurate to say a horse is a larger breed of a pony. They bred them to be bigger over the years since that made them able to carry more weight.

So was the Biblical passage saying “well she wasn’t so depraved to want the much larger horse cock, she was at least demure enough to only want donkey cock”? Or was horse cock smaller in those days so was it saying “horse cock wasn’t big enough for her, she desired men with the much larger donkey cock”?

This may have serious theological ramifications and needs more research.