The word "criterion" refers to a standard or principle by which something may be judged. Its plural form is "criteria."
"Datum" is a single piece of information, whereas "data" is the plural form, referring to multiple pieces of information.
A single desert plant is a "cactus," but when there are several, they are called "cacti."
"Thesis" is a statement or theory, with "theses" as its plural.
An "appendix" is a section added to the end of a book. Its plural, "appendices," refers to multiple such sections.
An "alumnus" is a male graduate, with "alumni" being the plural form for a group of male graduates or a mixed-gender group.
"Analysis" is the process of examining something in detail. "Analyses" is the plural.
"Medium" as a singular noun refers to a means of communication, while "media" is the plural form.
A single microorganism is a "bacterium," with "bacteria" being the plural.
An "ox" is a type of cattle, and "oxen" are multiple such animals.