The Hebrews lacked an appropriate word for “whale,” because they only had a 3,000 word vocabulary. Archer writes,
It should be observed that the Hebrew text of Jonah 2:1 actually reads dāg gādōl, or “great fish,” rather than a technical term for “whale.” But since Hebrew possessed no special word for “whale,” and since no true fish—as opposed to a marine mammal—is known to possess a stomach as capacious as a whale’s, it is reasonable to adhere to the traditional interpretation at this point. The only other available term, tannɩ̂n, was too vague to be very serviceable here, since it could also mean shark, sea serpent, or even dragon.).
Thus a “whale” is the best rendering of this Hebrew word.
 Archer, Gleason. A Survey of Old Testament Introduction (3rd. ed.). Chicago: Moody Press. 1994. 342.