Purportedly the first version was, indeed, horsemeat.. Now it's also made from beef, pork, and venison.
Cuts of meat are dry-salted in tubs (originally of beech or oak wood). Bay leaf, juniper, garlic and pepper are added to the salting mixture and the meat itself is sometimes also bathed with wine. The process is comparable to that used to make bresaola, including the ageing, which must be for at least one month. The pieces of meat, once ready, become a very dark red in color.