|The Queen Mary Psalter, 1310-1320. London, British Library,|
more HERE at the Medieval Animal Data NetworkBlog.
answer: They eat stones, nails and anything else they can swallow. No data about horseshoes however.
I should make a comment on humans eating weird things: It is called "PICA" and is often found in those in cultures where snacking on clay etc is an acceptable alternative in the days before potato chips etc. But we also find it with iron deficiency anemia, who lack minerals, and of course the mentally ill.