Sweetie Belle Derelle is a popular reaction image used in derailments on /oat/ back in 2011, and quickly became one of the most popular forms of derailment in Ponychan history. The term, "Derelle" is a portmonteau of the name "Sweetie Belle" and "derail." In the context of a derail, the targeted thread goes about a spamming of the reaction image, typically accomodated with a spamming of capital A's. In different context, the reaction image is now commonly seen exploited outside Ponychan's community as well, and at one point, the face was shooped on Rainbow Dash Network's logo.
The reaction image itself is a cropped color-sketch of Sweetie Belle, featuring derped eyes and a wide open mouth. This was originally part of a drawing on DeviantART by thedarklordkeisha that featured an agitated Rarity stitching fabric with Sweetie Belle behind her, drawn with the proverbial wailing "Derelle Face."
The first thread to undergo the derail occurred on June 19th, 2011, titled "Ponies?! IN MY OATMEAL?!"
Amonisis used the reaction face in the thread, and others began to spam and exploit it throughout the thread.
Where is it now?
The success of the meme later led to variations of derails involving the remaining CMC members, including Scootastare and ApplebloomOMFG. These derails were very common at its time due to the lack of enforcement on the "no derailing" policy over the summer of 2011.
Sweetie Derelle pictures are still fairly common place on Ponychan, especially the /oat/ board. However, they are viewed by Ponychan moderators as a derail, and are thus treated in much the same way as Spiderman, where use as a reaction image is acceptable, but derailing threads with them is grounds for a ban.
And Riot Control will get you if you post it. Riot Control doesn't like Sweetie Derelle.
After all, riot control AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA