[sandworm101]

Note that those are animal we generally hunt/eat. I'd bet this is tied to the language of the ruling/hunting classes of England, back when they spoke French more than English.

There are also some middle-ground words like "Shark". One goes fishing for "shark" like they would "fish" but it is more common to say "several sharks" using a plural as opposed to "several fish" using the singular. But "fishes" is still a word, which likely goes back to ruling classes who ate fish but generally did not hunt them as they would have deer.

[banannaise]

"Fishes" is a plural of a plural. You wouldn't likely say "two fishes", but you might say "all the fishes in the sea", referring to many groups of fish (much as you might refer to the "peoples of the world" referring to many cultures). Aside from that, I bet you're onto something.