Hartshorn is much used in Danish cooking recipes, esp. Christmas cookies (it's not really Christmas without it(-: ) It is sort of an old-fashioned baking powder, used as a leavening agent, but it tastes of ammonia when heated - but in Christmas cookies, all the gas can escape through the pastry and the taste of ammonia is avoided.

Hartshorn was made by crushing the end of antlers from a deer (although I think that is forbidden today.)

Apparently (my father told me this so I will not guarantee that it is absolutely true:D ), but apparently, hartshorn was used in medical treatment - to cure diarrhoea, dysentery, insect bites, fever and as a smelling salt.