BeevesThis one might surprise you, but the plural of "beef" is actually "beeves." It's rarely used anymore, but you might come across it in historical texts. Most commonly, we use "beef" as a mass noun, so it wouldn't have a plural form anyway (like "water" or "sugar").
Criteria
This word comes from Latin, and its plural follows the Latin pattern: "criteria."
Data
Another word from Latin, "data" is the plural of "datum."
Noses
This one seems pretty straightforward, but some might be surprised to learn that the plural of "nose" isn't "noses." It's simply "noses."
Syllabus
The plural of "syllabus" is "syllabuses" This one follows the standard way to form plurals for words ending in "us."
Stadia
The plural of "stadium" isn't "stadiums." It's actually "stadia." This follows the Latin pluralization rule.
Pennies
While it might sound more natural to say "pennies," the formal plural of "penny" is actually "pence."
Niblings
This is a relatively new word that is gaining traction. It refers to all of your siblings' children (your nieces and nephews).