"Why Wub Woo" is a catchphrase often associated with an exploitable image of Rainbow Dash from "My Little Pony:Friendship is Magic". The "Why Wub Woo" image macro frequently appears as a reaction face, and Rainbow Dash's face is often photoshopped on to various other characters.
The image was taken from the Season 1, Episode 4 titled "Applebuck Season" that aired on November 5th, 2010. It features the character Rainbow Dash making an excited facial expression while proclaiming eagerness to train with Applejack, and can be seen in the video below:
Soon after the episode aired, a templates of Rainbow Dash at the podium and Rainbow Dash's face with transparent backgrounds were posted to 4chan's /co/ board inspiring the creation of photoshopped images.
On January 22nd, 2011, a 1920×1080 wallpaper was posted to Deviantart by user ShelltoonTV. A "Why Wub Ponies" Tumblr page was made on May 31st, 2011. On June 12th, a thread was posted to Ponychan referencing the images.
The face has been represented by this emoticon: /)^3^(\