Humanity's space exploration ultimately took the form of billions of identical probes, capable of building anything (including astronauts themselves) upon arrival at their destinations. One lands in Equestria. Things go downhill from there.
Tracy needed somewhere to stay, how was he supposed to know that it was in another universe? Now he'll somehow have to hold down a job on Earth while living as a pony in Equestria. It's either that, or say goodbye to being human.