The “Mare Do Well” is a mystery and has protected wasteland for over 200 years.
She is a Pegasus Pony , in her armor she has protected travelers on the roads. Showing raiders that they are not safe from her justice. She met Blackjack in Thunderhead and told her and her friends to leave. But soon Mare Do Well realizes that she can't stop Lighthooves on her own.
In chapter 61,she revealed as Rainbow Dash and she has become a ghoul. Rainbow Dash's fight with Gilda ended when the former fell into a Balefire crater, and was left for dead by the griffin, were she was turned into a ghoul. Knowing she could not return to the Pegasus nation after being labelled a traitor by the newly emerging Grand Pegasus Enclave, Rainbow retreated to Thunderhead and Shadowbolt Tower (the local MOA hub) in the Hoofington region.
Using the extensive network of secret passages and rooms built into the city Rainbow Dash set up a base of operations for her to work from secretly, as she continued to try and help ponies both on the surface and across the enclave. After acquiring a set of prototype Mark IV Power Armor, Rainbow worked in disguise under the name of The Mysterious Mare Do Well and became something of an urban legend across the pegasus nation and small areas of the wasteland.