is this animal classified as a reptile, mammal or amphibian or any other?
Mammal
Platypus
mammal 
when did the duck billed platypus become extinct There is a threat of extinction but they are not completely extinct
wiki:25677052
Platypus [SEP] Introduction
The platypus ("Ornithorhynchus anatinus"), sometimes referred to as the duck-billed platypus, is a semiaquatic egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative of its family (Ornithorhynchidae) and genus ("Ornithorhynchus"), though a number of related species appear in the fossil record. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young. Like other monotremes, it senses prey through electrolocation. It is one of the few species of venomous mammals, as the male platypus has a spur on the hind foot that delivers a venom capable of causing severe pain to humans.