"I want to cum inside rainbow dash" is an overused quote used on /mlp/ argued to all be posted by either the same person, or by just some bored anons. These threads are only a margin above /b/reads, because all they really do is attract newfags to comment random shit about them. Contrary, to popular beliefs, they aren't good for stealth r34 requests, as the use of the word, "cum" is just going to bring the mods attention to the thread anyhow.
The post is then accompanied by an animated gif (that's right, you're supposed to post the animated one. A lot of idiots here don't know that.) of Rainbow Dash stretching her forelegs, then swinging her body forth to stretch her hindlegs. This has led to countless variations of ponies in the same pose, ready to be cummed inside.