Quite a few creature-names do not change in the plural...bison, cod, deer, fish, grouse, salmon, sheep, squid, swine, for example. Others have a normal plural form but still sometimes retain the singular format. "He kept ducks on the farm" and "They were at the lake shooting duck"
The plural 'fishes' is old-fashioned or biblical nowadays, but still exists. You might hear it at a rugby match involving Wales, where the crowd sings "Bloody great fishes are w(h)ales."