English includes many nouns that end in “-um” or “-on” and are pluralized by changing the ending to “-a.” They come from Latin and Greek respectively. Examples include “medium,” “datum,” “ovum,” “criterion,” and “phenomenon.” As with everything else, the language isn’t consistent; we have museums, not musea; morons, not mora; polygons, not polyga. I wouldn’t complain if the language regularized the endings of all these words. “Bacteriums” and “phenomenons” would sound weird at first, but we could get used to it.

What’s happening instead is that people turn the plurals into singulars. With some words, like “data” and “media,” the change has been firmly established. Others are substandard but turn up often, like “a bacteria” or “a phenomena.” Recently I saw a writer friend who should know better talking about “a criteria.” This doesn’t make the words any more regular, since double-pluralizing “medias” or “datas” is still frowned upon. It leaves us with words that are the same in the singular and the plural.
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