Rainbow Dash is the only Pegasus able to do a Sonic Rainboom in the show.

[HedonismBot 218]

36 minutes ago, Landi72 said:

Technically, if a pony were to do a special move (like a sonic boom) using assistive technology (like rocket boosters), it wouldn't quite be the pony completing the action. In that case, it would be the device itself which is powering the move, whereas the pony wearing it is just simply steering it. It's a bit different from actually having to use your own wings to power the special move you are attempting.

Well, Scootaloo specifically might not need the engine part - her scooter activities suggest that thrust is no problem for her. An upgraded version of Wind Sock's glider (from comic issue 81) could be enough. Get some lighter and stronger materials, replace the DaVinci-inspired wings for a real airfoil, and that should do it.

[swu24 8]

I wonder if Rainbow Dash is the only pony to do a sonic rainboom. when twilight did a unicorn version of that, it did the exact same thing only it followed her mane colors

 

[Kadeda 178]

I don't think it's just a speed thing. There are a lot of ponies that can fly just as fast or faster than her. Plus, she herself was able to do it as a kid and there's no way she got slower over the years so it can't be just speed. 

I think the reason that it's legend is because it's more of a "chosen one" skill than anything.

[Paleopony 690]

I think other pegasi have already done it before as hinted by Sonic Rainboom but they are probably dead, so I think Rainbow is the only pegasus that can do it that is still alive.