when sheep gains an incorrect -s when pluralised and fish an -es.

Actually, fish is the plural of fish when speaking of a single species of fish, but when multiple species are involved fishes is the "correct" plural. In Jacobean English fishes is perfectly OK. It appears numerous times in the King James version of the Bible, including the story about the fishes and loaves.

I've never heard sheeps, but I don't doubt that somebody might try to make sheep plural.

Ceci n'est pas un seing.