One of the more interesting aspects of the changes that take place in English from generation to generation is the fact that as spellings change to conform to modern usage, some of the old forms stick around with different connotations or meanings.
Two words for angel that came into English from Hebrew have plural forms ending in -im: cherub/cherubim and seraph/seraphim.
In angel lore, a seraph is a “fiery six-winged angel” who guards God’s throne. A cherub ranks just below a seraph and has two large wings, a human head, and animal body. A cherub is the guardian of a sacred place.
Seraph has not entered into general use, but in modern English cherub refers to the image of a pretty Cupid-like child with wings, or to the little faces with wings one sees as architectural decorations. A child with a beautiful, innocent face can be called a cherub. For these modern uses the plural of cherub is cherubs.
The earlier plural of brother was brethren, a form still seen in the King James version of the Bible and still to be found in sermons and some religious writing. It suggests spiritual kinship.
The plural fishes for fish has a kind of Biblical ring to it, as in the miracle of the loaves and fishes.
Pence as the plural of penny is still used in Britain while Americans say pennies.
The word dice is the plural of die: a cube with spots used in gaming. Die can also refer to an instrument used in manufacturing. The plural of that kind of die is dies.
Some other words with more than one plural form:
formula formulas formulae: The Latin plural formulae is often preferred by scientific writers.
index indexes indices: The plural indices has a specialized mathematical meaning (a number or symbol or expression written to the left or right of and above or below or otherwise associated with another number or symbol or expression to indicate use or position in an arrangement or expansion or to indicate a mathematical operation to be performed).
staff staffs staves: The plural staffs is the modern choice, whether you’re talking about a group of workers or a stick used as a walking aid. If you’re writing an historical novel, however, Robin Hood and Little John would fight with staves. The word stave occurs as a singular musical term. It is also the word for one of the strips of wood used to make a barrel. The plural of stave is staves.