Not much difference between the original unsprung mountain bikes and cyclocross bikes except that cyclocross bikes were modified street bikes and mountain bikes were modified kids cruisers.
In cyclocross the idea is to cross some rough terrain as rapidly (or efficiently) as possible. In mountain biking the idea is to cross rough terrain riding the bike.
Cyclocross bikes are better when you throw them over your shoulder and run up sections that are too steep and knarly to ride. Mountain bikes are better in moderately rough terrain where the fat tires can absorb a lot of the punishment.
Hybrid? What is a hybrid? That's just a marketing term for a fatter tired, heavier bike.