deer / fish / sheep
Nouns whose singular and plural look the same.
Deer, fish, and sheep keep one form for singular and plural: one deer, three deer; one fish, three fish; one sheep, three sheep. Deers is a mistake. Fishes is reserved for kinds of fish, or for the verb.
Examples
Usethree deer in the field
Usetwo fish on the line
Avoidthree deers in the field