Rainbow Dash doesn't know a lot about changelings. She knows that a hive of them tried to invade Canterlot. She knows that they feed off of emotions. She knows that they can disguise themselves as anypony. And she knows that, when she finds one alone and starving in the Everfree forest, helping it is the right thing to do, as much as she doesn't want to admit it.

After feeding off of Dash's conflicted emotions enough to regain consciousness, the changeling introduces herself as Butt. And, in a show of gratitude for saving her life, she offers to back Dash up.