Rainy Day Pony by Licorice Twist
Link - http://www.fimfiction.net/story/34724/rainy-day-pony

"This is a fanfic interspersed with the lyrics of  "Rainy Day Man" from Sailor Moon to show the growing relationship between Applejack and Rainbow Dash. Applejack is the faithful, loyal best friend who is always Rainbow Dash's shoulder, there through thick and thin no matter what. And throughout, the two begin to realize their feelings for one another."

Music used:
Rainy Day Man (1996) performed by Patricia Tollett, Sailor Moon OST distr. Kid Rhino