You can download and install new breeds from sites such as Mummy's Creatures and Creatures @ SeeYou7.Net. Species are a little more tricky. You get the grendel and ettin species with C3, and of course, you get the norns with DS and C3.
creatures caves is your #1 resource for the creatures artificial life game series: creatures, creatures 2, creatures 3, docking station, and the upcoming creatures family.