horses

Of male animals the genitals of some are external, as is the case
with man, the horse, and most other creatures; some are internal, as
with the dolphin. With those that have the organ externally placed,
the organ in some cases is situated in front, as in the cases
already mentioned, and of these some have the organ detached, both
penis and testicles, as man; others have penis and testicles closely
attached to the belly, some more closely, some less; for this organ is
not detached in the wild boar nor in the horse.