You know how when a dog is sporting red rocket you can see his pinkish dong emerge from that little furry sheath that normally hides it? What’s that sheath called?
In horses, it’s called a penis sheath.
Rather unimaginative, but clearly descriptive.
Picture at http://www.vetmed.wsu.edu/cliented/anatomy/dog_ug.aspx labeled, as noted above, prepuce.