The most common way for an English noun to form its plural is by adding an s: bed, beds; toaster, toasters; suitcase, suitcases.
Another common way for an English noun to form its plural is by changing a vowel sound: tooth, teeth; mouse, mice; foot, feet.
I just now noticed that there is a reasonably common unhyphenated, uncapitalized English word that forms its plural by doing both: It changes a vowel sound and adds the letter s. This is the only plural for this word given in three dictionaries that I’ve checked, including the Merriam-Webster Tenth Collegiate. The word isn’t archaic or obscure or in a dialect or anything like that, though most of us probably don’t have occasion to use it very often.
I can’t think of any other word in English that does this. So I’ll pose it as a puzzle: What’s the word?