Adam Clarke on Genesis 1:21

And God created great whales - התנינם הגדלים hattanninim haggedolim. Though this is generally understood by the different versions as signifying whales, yet the original must be understood rather as a general than a particular term, comprising all the great aquatic animals, such as the various species of whales, the porpoise, the dolphin, the monoceros or narwal, and the shark. God delights to show himself in little as well as in great things: hence he forms animals so minute that 30,000 can be contained in one drop of water; and others so great that they seem to require almost a whole sea to float in.