Most countable nouns have distinct forms for the plural. An important exception is the names of a few animals:
one sheep, two sheep
one moose, two moose
Sometimes, a native speaker may feel comfortable using either form:
one beaver, two beaver
In some cases both forms are possible but one is more usual:
one fish, two fish (but two fishes is not wrong).
In most cases, though, the regular plural is needed:
one cat, two cats.