Take a look at this

[True Federalist (진정한 연방 주의자)]

Can anyone explain why they all have odd plural forms?  Deer, elk, and moose are all very nice but deers, elks, and meese are not apparently.

In the case of moose, it's in part because it's an import from Algonquin well after English was adding new strong nouns. If it were to be pluralized it would be as mooses. While I am not clear on why English has this rule, as a general rule, game animals in English are invariant nouns that don't inflect as plural.