I finally finished AlonPony for Source. Doesn’t he look awesome?
 
So, the story is that Alonguy was called in by Celestia to defend Equestria from a possible attack from the Dark God, Malum.  
Since Alonguy was the only one who was effective at fighting Malum’s forces, Celestia summoned him to her world and transformed him into a pegasus pony to defend their world from Malum until the threat is gone.
 
Even though at first he’s rather reluctant to accept and wants to get it over with as soon as possible and return to his own world in his own body, he soon discovers that being the hero of Equestria isn’t so bad.