Wikipedia gives a pretty good overview of the rules. Based on the information there (don't cite it in a paper, but it's good enough for our purposes), nouns ending in -o are accompanied by nouns ending in a sibilant sound, which is far beyond the scope of this answer. But at any rate, here's the relevant sections from the Wilipedia article on English plurals:
Where a singular noun ends in a sibilant sound —/s/, /z/, /ʃ/, /ʒ/, /tʃ/ or /dʒ/— the plural is formed by adding /ɨz/. The spelling adds -es, or -s if the singular already ends in -e:
kiss kisses /ˈkɪsɨz/
phase phases /ˈfeɪzɨz/
dish dishes /ˈdɪʃɨz/
...
With nouns ending in o preceded by a consonant, the plural in many cases is spelled by adding -es (pronounced /z/):
hero heroes
potato potatoes
volcano volcanoes or volcanos
However many nouns of foreign origin, including almost all Italian loanwords, add only -s:
canto cantos
hetero heteros
photo photos
So in summary, add -es if:
- the word ends in -(consonant)o (usually), unless it isn't actually an English word and we're just borrowing it.
- the word ends in a certain kind of consonant sound, simplified to most uses of "s", "z", "ts", "sh", or "ch" (plus, I think, a couple others, but those cover most cases).
The word "police" is rather special: It has no singular noun form. Something like that police over there is securing the scene would be incorrect. One would always construct sentences in the plural form like so:
The police are out in force today.
Anything done by the police will reflect on them.
Other words that take no singular form would include pants, trousers, scissors, and clothes.
Confusion arises because "police" is also used as an adjective. Consider these sentences:
A police department is housed in that building.
The police chief was highly visible at the town meeting.
In these two sentences, we are not speaking of "a police". You could easily remove the word from both sentences and they would make sense semantically and grammatically. Instead, the word describes the department or chief. It gives us context.
"Police" also has a verb form. You may encounter it like this:
The Boy Scout troop must police the area before they leave to remove any trash.
The verb means "to investigate, to search, to clean up". This certainly does fit in with a subset of the duties of a police department.
Best Answer
There is no rule determining whether or not a noun's plural form is the same as its singular form. As the comments point out, many of these words are words for animals. Another trend I have noticed is that loanwords from Japanese often don't change in the plural (two kanji, three Pokemon, ten anime), though sometimes they do (six ninjas, eighty futons).