It took a very long time, but when both Granny Smith and Twilight gave their approval Spike was free to become the faithful student (he really didn't like the term sidekick) of Batmare.
As for what he wanted to call his new superhero identity, that took a lot of thinking and brainstorming. But Spike eventually settled on the name of "Robin", after observing how graceful and majestically the namesake bird flew through the sky.
And, as any superhero worth his salt will tell you, regardless of whether you're a leader or a sidekick you have to have a suitable costume. One that delivers the message you want to send to your enemies, and one that you can work with. It also helps if the costume is well designed. Spike decided on a dark red vest with black stripes, and a small capital "R" printed onto a badge on said vest. He would have a black cape and matching utility belt, just like Batmare. Rather then cover his whole face Spike simply had a small black mask that would obscure his face and upper nose, one that would make him look like he was glaring down at his opponents. And to top it off, he would have bright white gloves on his hands, and the same pair of black boots that Batmare wore.
Rarity was all too happy to design the costume for Spike, who had to be practically dragged out of Carousel Boutique by Applejack since he wouldn't stop starring at Rarity.
Applejack quickly laid down the ground rules of being a superhero, and made sure to specifiy that it wasn't all about making a big entrance and picking up chicks. Spike proved to be a fast learner, and despite being a bit cocky he quickly adapted to his new life. Before long Batmare was no longer alone, for whenever she went out Robin would be close behind. The two came to be known as "The Dynamic Duo", but they stayed mostly out of the spotlight.
Of course, Robin had no interest in simply playing second fiddle. And with the money he had been saving as Spike, he purchased an experimental motorbike. The intent was to use it to get to places that The Batmobile was too wide to fit through. Sometimes he was heard to boast that his Robin Racer was faster than The Batmobile. Batmare had no interest in proving him wrong.
The Batcave was expanded, and rumors even circulated that Batmare and Robin were planing to one day build a Batsub and a Batcopter for sea and air operations.
Finally, after about two months of being the center of attention, the dynamic duo made one last noticeable contribution to the wide realm of Equestria.
"And so, it is with great pride that I hereby dedicate this offical "Bat Signal', to contact our greatest defenders when we need them most." Mayor Mare said proudly, unveiling a spotlight at Town Hall which had the same Bat Symbol that could be spotted on Batmare's costume. The Bat Signal actually drew inspiration from the time Batmare and Robin had took down a nostorious jewel thief, and had left the crook tied to a spotlight. When the police had found him they were quick to notice how, from the way he was positioned, he matched the Bat Symbol almost down to a T.
"Now, without further ado, here's District Attorney Fancy Pants!" Mayor Mare said "Our special guest who has been invited to test The Bat Signal. Mr. Pants, please do the honors."
"With pleasure." Fancy Pants replied, and flipped the switch. The auidence looked up into the night sky, and saw The Bat Signal shining brightly in the night sky.
From a distance Batmare and Robin watched the public display, and smiled.
"Do you think we did good Batmare?" Robin asked.
"Of course we did Robin." Batmare replied "Glad to have you with me."
And with that, the dynamic duo raced off into the night. Ready to help Equestria if ever the need should arise.