Put simply, Alphys Auris is an otaku lizard dinosaur. You don't understand the extent of her obsession until you realise she spends a majority of her waking hours watching anime with her girlfriend Undyne, and occasionally Neko.
Using strange methods, she extracted the substance DT from humans and injected it into dying monsters. The result was the monsters lived for a while, then they all started melting into each other. The final result was huge white "amalgamates".
Her UNDERSWAP counterpart is the splitting image of Undyne, if Undyne were a short, stocky, yellow lizard.