Is the biblical Leviathan a plesiosaur ?

No. Plesiosaurs were extinct millions of years before anyone thought about leviathan.

Probably not.  In Job we can read that Leviathan has claws that he can spread on the mire, so he is probably amphibious.  Maybe he is a large crocodilian - or was.

No - it's a mythical creature - like 'The Kraken' or 'Medusa'.

The bible is no more real than any other book of myth or superstition - and, in fact, plagiarizes heavily from older stories.

Maybe it was a giant squid.

The Leviathan is unlike any other creature in the world, like the other 2 legendary beasts, Behemoth and Ziz. There is no dinosaur that can even hope to rival any of these 3.

A person can better know the true meanings of Bible verses by reciting the rosary of the Virgin Mary every day with care and sincerity.

I suspect that Leviathan in the Bible is just a representation of the DEVIL.

Jesus came in order to destroy the works of the Devil.  That evil dragon has no power over those who give him no power in their lives.  He has no power over born again Christians in the Name of Jesus Christ the Saviour, who has ALREADY DEFEATED the devil and his cohorts, and made a show of them, openly triumphing over them in the Cross. (Colossians 2: 14 & 15) 

Leviathan is a
n animal usually associated with water, apparently some form of aquatic creature. At Job 3:8 and 41:1, it seems to refer to the crocodile or some other aquatic creature of great proportions and strength. At Psalm 104:26, it may be some type of whale. Elsewhere it is used figuratively and is not identifiable with any one animal.​—Ps 74:14; Isa 27:1.