HiE, gender bending, ponification.

Derek, a human, chases Rainbow Dash through a portal. Twilight is nervous that she is going to get in trouble for bringing a human to Equestria, so she tries to disguise him. The spell doesn't go as planned, and Derek wakes up as a girl.

To complete the disguise Derek needs to absorb pony genetics. Unfortunately he can't go home until he has absorbed enough pony genetics to become full pony. Eager to get back home he wonders if the end justifies the means. Will Derek ever see Earth again?