Boyd’s Bible Dictionary:
The Hebrew original translated “great whales”
(Gen. 1:21) is used of “serpents” (Ex. 7:9; Deut. 32:33), and of the “crocodile” (Ezek. 29:3; 32:2). The name belongs to sea monsters (Job 7:12; Isa. 27:1). It is thought that the shark of the Mediterranean is meant (Jonah 1:17; Matt. 12:40).
Concise Bible Dictionary:
The word tannin (Gen. 1:21; Job 7:12; Ezek. 32:2); and κῆτος (Matt. 12:40); refer to any sea monsters, without defining any particular one. In the case of Jonah the Hebrew word is dag, or dagah, a fish; it may not have been a whale: sharks have been known to swallow men entire, and whatever fish it was that swallowed him, it was a miracle that preserved him alive, and caused him to be safely landed on shore again. It is only by denying the miracle that any difficulty arises. The Hebrew word tannin, is also translated in the AV “dragon,” “sea monster,” and “serpent.”
Humpback Whale
Strong’s Dictionary of Hebrew Words:
Meaning:
from an unused root probably meaning to elongate; a monster (as preternaturally formed), i.e. a sea-serpent (or other huge marine animal); also a jackal (or other hideous land animal)
KJV Usage:
dragon, whale. Compare 8577