There is little doubt that dinosaurs once roamed the earth. The fossil record gives clear evidence of their existence. Many theories have been put forward to explain their disappearance, but not even the evolutionists have come up with any agreed authoritative conclusion.
Dinosaurs would have gone into the Ark with Noah because we are told that all varieties of animals were included. The earth underwent major environmental changes as a result of the flood, and it is very likely that many animals (including dinosaurs) could not cope with the new environment and eventually became extinct as a result.
The Behemoth and Leviathan, both referred to in the Old Testament, may well have been varieties of dinosaurs.