The word pea was originally pease in the singular and peasen in the plural. For example, a 15th-century cookbook has the following recipe for what we would today call pea soup:
Pease also functioned as a mass noun, like bread or oatmeal.
(As a side note, the above quote is pretty funny if you read it in a sarcastic voice, but the original context is a religious anecdote with the moral of cibus moderatus et uniformus causat sanitatem et puleritudinem corporalem: a moderate and uniform diet produces a healthy and cleansed body.)
Eventually, speakers understandably interpreted the -s in pease as the plural suffix rather than just a sound in the original Latin pisum/pisa and Greek πίσον, and the English singular pea was born.
A fifteenth-century cookery-book. Harleian Collection 279. (~1420). ↩︎
pēse. Middle English Compendium. (2019). ↩︎