Colossus is a generic term to refer to any gargantuan, mutated fauna in the Shadowmark. Some of the animals and creatures in the Shadowmark survived The Cataclysm and some of those ended up twisted and corrupted by the toxic energies of the Cataclysm.
The Colossi are those who ended up with massive physical size. Most people will go through their entire life and never see a Colossus with their own eyes. Some, on the other hand, have seen the terror and destruction these monsters can wreak upon settlements.