I've been sitting on this idea for a while and decided to go through with it. After watching the episode The Transformation I thought of a few head canon origins of the shapeshifting fairy species and I ultimately decided on blending Sci Fi with Fantasy for this.
I'll cut to the chase with this, my head canon is that Penny's race is a slightly evolved descendant of a genetically modified version of an alien race that tried to colonize Earth 2.5 Million years ago. This planet orbits Gamma Piscium, a prominent star in the Pisces constellation, and 2.5 million years ago it harbored an alien civilization slightly more advanced than us. Just like us they aspired to have a colony on another planet as it served as a long established sci fi trope in their culture and many experts on their planet viewed it as the only way for them to avoid extinction. They have long since discovered a long list of exoplanets in the stars surrounding them, including the Sun, and eventually narrowed them down to the most plausible candidates for a colony. Out of sheer coincidence they picked our solar system and later Earth itself.
The plan was to send these genetically engineered blank slate colonists (think if a human had the capabilities of a stem cell) at near light speed in cryogenic pod ships to Earth and had two plans in place based on wether or not the prospective colony planet had complex life. If the planet was inhabited: the idea was for them to take on the physical traits of the dominant organism in the environment they landed in as well as be able to survive in an atmosphere and level of gravity that's different than the home world. Then the home world would send a capsule ship with instructions for them to fully colonize the host planet by being able to evolutionarily one up whatever race dominated the other planet. If uninhabited: the colonists would sadly be doomed.
Earth was the first planet they tried this with since they had only recently developed the technology needed for such a feat. They had no other terrestrial planets in their system to colonize and their moon could only support a small settlement that was completely dependent on regular shipments from the planet. The public cheered on the progress as they saw that it could be their chance to live on another planet. However, the race was only able to send a few waves of the colonists because a couple of decades after they sent the last colonist pod a gamma ray burst sterilized the planet. This meant that the colonists heading to Earth were what remained of this race, and were a simplified genetically modified version too.
This was created in Planet Maker: planetmaker.wthr.usen.wikipedia.org/wiki/Gamma_Pi…