As I said, strictly speaking, the word kudos, or “glory,” is singular. Over time, those unfamiliar with its Greek root mistook it for a plural, leading to what’s called a “back-formation”– in this case, kudo.
Kudo may raise some folks’ linguistic hackles, but two other respectable words arose this way. In Middle English, pease (as in “pease-porridge hot”) served as both the singular and plural for those little green things, deriving from the Greek word for same, pison. Many people assumed, therefore, that the singular of pease must be pea. Something similar happened when Anglo-Norman cherise wandered into Middle English. Many assumed cherise was plural, and therefore one of these tangy fruits must be a cherry.