About
I Want to Cum inside Rainbow Dash is a satirical catchphrase aimed at mocking fans of the animated series My Little Pony: Friendship is Magic, specifically cloppers (fans who masturbate to explicit art of the show). However, as time went on, people started to repeatedly post threads about ejaculating inside Rainbow Dash, usually in jest, giving the phrase a reputation similar to daily dose. The phrase is commonly accompied by an image of Rainbow Dash.
Origin
In an /mlp/ thread on 4chan in September 2012, an anonymous user created a thread saying that he would create a song out of posts that ended with dubs (e.g., XXXXX22, XXXXX99, etc.) One post with the phrase “I want to cum inside Rainbow Dash” got dubs and was listed as a verse in the song. Although the originally posting anon didn’t return to the thread, another user came, arranged, and produced the song known as “Graaaaavy Train (The /mlp/ Anthem).”[1]
Spread
Various I Want to Cum Inside Rainbow Dash threads have appeared and currently appear on /mlp/ and have been seen in other boards on 4chan including /jp/ (Otaku Culture) and /m/ (Mecha), and the phrase tends to occur in comments and following posts whenever the most frequently used image of Rainbow Dash stretching appears in throughout the Brony fandom.
[1]SoundCloud – Graaaaavy Train (The /mlp/ Anthem)