Had he been able to produce one, would it have been a ferret? Since it appears that a person's Patronus takes on the same form as their Animagus. I realize that a ferret might not have been Draco's Animagus form but rather Moody's choice of animal in GoF, but are there any mentions of what form Draco's Patronus could have taken?
Per JK Rowling's original website, it's not known.
Q. What is Draco Malfoy’s Patronus?
JKR: As of the end of Half-Blood Prince, he has no idea how to produce one, so nobody knows. You must remember that the ability to produce a Patronus demonstrates an advanced level of magic not routinely taught to young Hogwarts students (hence the surprise of the prospective members of Dumbledore’s Army when they find out that Harry can make a Patronus).
Given his lack of need to produce a Patronus charm (it's very specialised and not especially useful) and the fact that his formal education ended shortly afterwards, it seems likely that he simply never learned to form a corporeal Patronus. Any speculation on the matter is simply that.