When the school chooses an animal as its athletic nickname, usually in the plural or as a collective noun for a group of that animal, then typically, the school has that animal (in the singular) as its mascot, either specifically named with a proper noun or generically referred to without a proper noun.
When a school chooses an animal for its athletic nickname, it often uses the plural or collective noun form of that animal to refer to the team. This usually means the school has that specific animal (singular) as its mascot, either with a specific name or a general term without one.