User blog comment:Zaid1959/The first beast/@comment-5619706-20140913020154

Well they made the Leviathan (a beast of the sea) fluid, so I would think the Behemoth (as a beast of the land) would be unyielding and invulnerable in the way that literally anything you use against it would shatter or bend, while the Ziz (beast of the sky) is invulnerable in terms of being incorporeal or really light like air.

Basically, a trio of invulnerable beasts--Behemoth the unyielding, Leviathan the undying, and Ziz the untouchable.