It's like this.
Many biologists have speculated about the existence of a mammal - well, they think it's a mammal as it ticks at least some of the boxes - with the beak and webbed feet of a duck (bird), fur and rudimentary mammary glands (mammal), which lays eggs (bird/reptile), and which has poison glands and a means of delivery for same (reptilian).
Such a creature, which appears to have been created by assembling the leftovers from at least three phyla, feels like an experiment made by the God of Evolution, but the truth, at least on Discworld, is simpler than that.
The old saying is that a camel is a horse designed by a committee. There is some experimental truth in this, as the Faculty, when visiting XXXX during its creation, decided to help out the Creator and improve on his drawings, by hi-jacking his pigments and drawing stick, and designing him a nice jolly waterfowl: so it can justly be said that the duck-billed platypus is a duck designed by a committee - of senior wizards. Nobody has ever asked the platypus what it feels about all this, but informed speculation has it that it is an unhappy and somewhat confused creature.