In English we have a number of animals that officially do not get pluralized. The most well known ones are fish, deer, and sheep. It's supposed to be true, too, of antelope, buffalo, manatee, and zebra. When pluralizing any of these animals it is supposed to refer not to just multiple animals, but multiple groups or types of that animal. When you say, "I saw five fishes," it is supposed to mean not that you saw five individual fish, but rather five different types of fish (like goldfish, minnow, bass, salmon, and catfish). However, this rule is becoming less strict and it is not uncommon these days to hear all these animals pluralized when it is just referring to multiple individual animals.
By the way, it is the same with mass/uncountable nouns. "Five rices," means five types of rice, not five grains of rice nor five servings of rice.