By far, this topic has received the most attention and comments on this blog. Many, if not most, cases of Horner's syndrome are idiopathic, meaning that no discernible causes can be found. Again, Horner's syndrome is not a disease per se, but rather a constellation of clinical signs that make up the syndrome: ptosis (or drooping eyelid), enophthalmos (or sinking in of the eyeball), miosis (constricted pupil), and prolapsed nictitans (presence of 3rd eyelid covering the eyeball more than usual).
When this young Cocker Spaniel was presented to me yesterday for an eye infection, she exhibited all signs of Horner's mentioned above. By default, because she's a Cocker Spaniel, I had a good look deep into her ear canal and lo and behold she had pretty convincing evidence of an ear infection, a ruptured ear drum, and likely a middle ear infection. X-rays of her middle ears were iffy, as they often are (CT is better for these little structures). We sedated her, flushed her ears with saline, cultured the "gunk", and sent her home on topical and oral antibiotics, and analgesics.
In this case, otitis media (an infection of the middle ear) is the likely culprit causing Horner's syndrome.
Not the Cocker Spaniel in question! The affected eye is evident.