It does seem odd to employ "cornucopia" in the plural, as when I referred to my old pension consulting colleagues as "cornucopias of wit." On the other hand, just as I can picture a goat’s horn with fruits, grains and vegetables spilling from it, so I can picture multiple horns overflowing with multiple bounties.

Cornucopia comes from the Latin. If a cornucopia is a horn of plenty, it seemed reasonable to conclude cornucopiae are horns of plenty. I likened the word to alumna, a female graduate of a college or university, with alumnae being the plural (e.g., the alumnae of Smith College ).

So much for my presumed analogue: when I typed cornucopiae, Microsoft’s squiggly red line materialized, like an angry dog nipping at my heels. This sent me scurrying to the online dictionary, which informed me “cornucopias” is in fact the plural form of the word. There was a caveat:

Some writers maintain that this word should be written, in the singular, cornu copi[ae], and in the plural, cornua copi[ae].

This, however, was too recherché even by my standards.

http://onlinedictionary.datasegment.com/word/cornucopia