Shadowbolts

The Shadowbolts are Nightmare Moon's shape shifter form

Friendship is Magic
They appear at the part where Rainbow Dash was going to fix the broken wooden bridge so her friends could cross. The Shadowbolts try to make Rainbow Dash join with them, but Rainbow Dash was loyalty to her friend so Rainbow Dash fix the broken wooden bridge so her friends could cross to the other side. Then the Shadowbolt turn into Nightmare Moon's shadowy magic form.

Luna Eclipsed
In the episode Rainbow Dash made a costume of the Shadowbolts with her to wear for Nightmare Night, and Rainbow Dash went off scareing everypony by using a thundercloud.