I hadn't heard that story before, waysider. I thought those horns were related to harvest and we were supposed to be harvesting from our sowing (but then wouldn't we have been taking from them?) - must look it up. I'm fairly sure the plural should be "cornua copiae," cornua being the plural of cornu, copiae being the genitive, for "horns of plenty," at least in the two-word standard Latin. According to the one-word usage, I don't know.
Hope this doesn't come across as being too pedantic, because I know I can be that, eg, wondering why people who say they are Greek scholars use criteria/phenomena as singular nouns. Normal people who don't make those claims might never have had a reason to look the words up. Sounding more grumbly than I really am, I think. Proper use of criteria should not be a criterion for judging true love and sincerity, I tell myself! I know that to be true, as well as "who am I that i should judge."