2nd generation pets are just what they sound like: 2nd generation. When you adopt, it only shows a picture of the two "parents" (two already existing Chichen Smoothie pets). The litter that results is a mixed combination of the two parent pets. They should look something like this:
and
2nd generation pets can come in any species. There are also 3rd generation pets, where one of the parents (or both) are a 2nd Generation pet.
It has been frequently suggested that CS users be able to "breed" their own 2nd or 3rd generation pets. Tess and Nick, owners of CS, have said multiple times that this will never happen. The way CS is coded to work, it would be impossible. CS artists would literally have to draw thousands of pet collorations. If you want to breed pets, it is suggested that you go to a site such as Wajas. They are coded differently in a way that allows pets to breed.
And pancakes.