Use ending -es if a noun ends in -o.
But: add only -s to words of foreign origin or abbreviated words ending in -o.
If a noun ends in -y that is preceded by a consonant, change the -y to -i and add -es.
But: if -y follows a vowel, just add -s.
Some nouns that end in -f or -fe change the -f or -fe to -v and add -es.
But: some other nouns ending in -f or -fe just add -s.
Names of some creatures do not change their forms in plural.
Remember plural forms of the following words:
fish - fish
deer - deer
sheep - sheep