What are some of the different forms of creating plural words other than adding an S ?

Add an "es"

In addition to adding "s" or "es," in some cases the plural is formed by changing the suffix - this is often true for words that come from Latin or Greek. Examples of single and corresponding pural words from Latin include:

alumnus, alumni
alumna, alumnae
curriculum, curricula
focus, foci
genus, genera
opus, opera
datum, data


And from Greek:
criterion, criteria

Other exceptions:
child, children
person, people

And purists insist that the plural of octopus is octopodes, although octopuses is also accepted.

Finally, there are some words that are the same in both singular and plural:
sheep, fish, deer

ADD s , es or ies.

Easy one **** ox = oxen.

Hard one **** wonderkind = wonderkinder

There must be others!
How are these explained ?