What is the platypus a mixture of?

The platypus is not a mixture of anything. It is an entirely self-contained mammal, specifically a monotreme, which is an egg-laying mammal. It happens to have a bill which some believe resembles that of a duck, but is in fact much broader than a duck's bill, and which is equipped with electro-receptors with which it can locate food. It also has webbed feet with retractable webbing to expose its claws for digging - again, nothing like a duck's feet. It has a flat tail which some believe to be similar to that of a beaver, but it is not.

The platypus is a monotreme, one of three extant species, the other two being the short beaked echidna and the long-beaked echidna, of which there are four recognised sub-species. Albeit bizarre, the platypus is simply a creature of adaptation.