Rainbow Dash suffering brain trauma so bad, she has been mentally regressed to the mind of a small child.
Scootaloo finding out about Rainbow Dash's fate the hard way. Imagine seeing someone you looked up to and idolized for years, reduced to being less mentally mature than you are.
Applejack's brief inner-monologue sums up how devastating it must be for the entire mane six, to see their once proud and boastful best friend, mentally reduced to a small child.
Rainbow Dash's completely devastated reaction to finding out she has been physically regressed to a small child, in the alternative sequel Raising Rainbows.
