A childhood belief about general

As a kid, I was given a book in which I first read of the unusual egg-laying mammal of Australia, the duck-billed platypus. Funny thing was, I was sure I'd heard the word "platyous" before and envisioned that a usual platypus was some kind of much larger animal, and probably a more usual mammal (perhaps a placental mammal, although I didn't know that term at the time). A usual platypus, so I had an idea, was probably close to as big as an elephant, or at least as big as a hippopotamus. So, I assumed that the name "duck-billed platypus" was given to distinguish that small and unusual egg-laying mammal from a more more usual and larger platypus. But gradually I learned that the duck-billed platypus is in fact the only kind of platypus that there is, something I still have to consciously remind myself of on hearing the word "platypus". And it is still quite a mystery just how my mind concocted that belief in a much larger "usual" platypus.