Frisk was wandering around Waterfall, but they decided to sit down and enjoy the environment around them. While they're relaxing, a flower adjacent to them sprayed some sort of unknown pollen at them(probably tainted by one of Alphys' failed chemistry experiments). At first, Frisk didn't notice the metamorphosis until they looked at their transformed body, especially their tummy. Frisk doesn't mind about their dragon form, they now think they're larger than life, or in this case their soul. Speaking of, Frisk's human soul became a monster soul after the transformation, so pretty much their adventure ends here. (Or Alphys can probably turn Frisk back into a human, as well as their soul. You decide.) Frisk, the former human, can happily live in the underground for the rest of their life.

Art © Me
Undertale © Toby Fox