ANSWER: Dinosaurs are simply large reptiles, many of which are the largest known animals ever to have existed on the planet, that have lived a long time. There are at least three specific places where they may be mentioned in Scripture. One of these references in the book of Genesis.
The other two possible references to dinosaurs are in the book of Job, which some commentaries state is the only Old Testament writing that possibly mentions these large reptiles (Willmington's Complete Guide to Bible Knowledge – Old Testament Survey, etc.).
Dinosaurs existed before the flood and to a lesser extent after the flood. The record found in fossils indicates that the present planetary ecosystem is incapable of producing and supporting large bodied reptiles and mammals.
Apparently, before Noah's flood destroyed the antediluvian ecosystem, the earth could produce large bodied long living reptiles that we call dinosaurs (a word which means a fearful or monstrous lizard). The word of God states that not only did all humans, except eight, were destroyed by the flood, but also the earth itself was wrecked (Genesis 6:13).
The first possible mention of dinosaurs in the Bible is in Job 40. God, speaking directly to the patriarch Job, challenges him regarding a mysterious beast known as the Behemoth.
Now behold the behemoth, which I made along with you; he eats grass like an ox; See, now, his strength is in his loins, and his force is in the muscles of his belly. He moves his tail like a cedar; the sinews of his thighs are knit together. His bones are like tubes of bronze; his ribs are like bars of iron. (Job 40:15 - 18, HBFV, see also verses 19 to 24)
The second possible reference to huge, now extinct dinosaurs is in Job 41. God, again speaking directly to Job, challenges him regarding his ability to catch or tame a creature referred to as Leviathan.
Can you draw out the leviathan with a hook, or hold down his tongue with a cord? Can you put a ring into his nose, or pierce his jaw with a gaff hook? . . .
Will you play with him as with a bird? Or will you bind him for your maidens? . . . (Job 41:1 - 2, 5, also see the rest of the chapter).
The third possible reference to these large reptiles is in Genesis 1:21 where it states that God created "great sea creatures." Many Scriptural scholars feel that these references are talking about the hippopotamus, crocodile, whale or other large known animal. Other scholars of a more scientific mind suggest that these verses are actually referencing dinosaurs.
The literal meaning of these references is not clear and either side may be correct. The fossil record indicates that organisms existing thousands of years ago (before Noah's flood) grew large. Examples of these are giant cave bears, giant sloths, giant dire wolves, and so on. From a biological point of view, many reptiles also have the unusual proclivity of getting larger as they age.
Will dinosaurs exist again to roam the earth when God creates a new heaven and a new earth (Revelation 21)? If he recreates all extinct creatures they will return. Some people have postulated that these large creatures will not be in the New Heavens and Earth because they are not mentioned as being alive during the Millennium (Jesus' reign on earth for 1,000 years).
Those who make the above argument forget that God's new creation will be first cleansed by fire (2Peter 3:12 - 13). There is nothing in the Bible to indicate that God could not repopulate the earth with some large reptiles like dinosaurs if he desired.