Kudos is not plural, and there is no such thing as one kudo, at least in the original Greek.
The word is Greek kydos, "glory" or "fame." Etymonline says it was mainly a poetic word, found chiefly in Homer's Iliad and Odyssey, and that it meant glory won in battle. Etymonline says it may literally mean "that which is heard of" and may come from a root meaning "to see, observe, perceive." Both are tentative.
English took it up late. Etymonline dates it to 1799 and says it was probably university slang. Merriam-Webster gives 1831 as the first known use, and Grammarphobia reports the same year as the earliest Oxford English Dictionary citation. The usage guide Merriam-Webster's Dictionary of English Usage says the word was "dragged into English" as British university slang. The two dates differ, and the sources do not explain why.
The trouble started with the final letter. In Greek, the s is part of the word. English speakers saw a plural ending and acted accordingly. Etymonline records the back-formation kudo from 1936, while Grammarphobia says the Oxford English Dictionary's earliest example is from 1941, in Jack Smiley's Hash House Lingo. Merriam-Webster says it was a count noun by the 1920s.
Grammarphobia suggests that a change in pronunciation may have helped. Speakers moved from "kyoo-doss" to "kyoo-doze," which sounds more like a plural. In the 1920s the praise sense began to be used as a plural count noun, and Time magazine may have helped spread that.
Usage guides are not in agreement. Fowler's says kudos has no plural in standard usage and is not used with "a." Grammarphobia reports that of ten standard dictionaries its authors consulted, only three accepted the singular "kudo," and two of those came from the same publisher. Merriam-Webster now lists "kudo" as a count noun.
So "kudo" is a back-formation that is now common but still contested. Etymonline calls it "barbarous." The safest use is the original one: kudos as a single mass noun, like glory.