Some nouns have same singular and plural forms or have plural ending (-s) 
Singular  Plural    Singular  Plural 
boar boar/boars   reindeer reindeer/reindeers
buck (animal) buck/bucks   roe roe/roes
Eskimo Eskimo/Eskimos   Sherpa Sherpa/Sherpas
Karen Karen/Karens   shrimp shrimp/shrimps
mackerel mackerel/mackerels    trout trout/trouts
quail quail/quails   whale whale/whales