An uninflected plural, zero plural, or invariant plural is a plural noun that is identical to its singular form and is therefore an irregular plural.
Examples in English include:
Animals - deer, fish (usually), rhinoceros, sheep, swine, etc.
Also some Latin plurals, e.g. "species",
Some words have both an uninflected plural and an inflected plural; in the vast majority of (all?) cases this is a regular plural that is with plural -s. Examples?
Learners may over-regularise plurals and say things like *"sheeps", etc. They just have to be learnt.