Sonic Rainboom, created March 28th, 2012, is a reference to both a phrase and an image that was immortalized as the example game on the past version of the Drawception homepage; perhaps because every panel provides a new example of the misinterpretation so dear to the site. It also led to two of the arguably most famous drawings on Drawception.
The phrase itself originates from My Little Pony: Friendship is Magic, and refers to a prestigious stunt performed by talented pegasi. In-universe, there is a group of pegasus ponies modeled after the US Airforce; a Sonic Rainboom is created under similar circumstances as an aircraft creating a Sonic Boom, but it results in a rainbow. (Pegasi in the series can influence weather.) "Hadouken," as used starting in the third panel, is an attack from Capcom's Street Fighter Series, but it is worth mentioning that its initial use in the game is essentially erroneous; as in reality a hadouken is a horizontal projectile--likely, it was confused with "Shoryouken," which is an uppercut similar to to Sonic's in the second panel.
Since the original game, the phrase has provided lots of room for interpretation, as artist's renditions have ranged from Sonic the Hedgehog, Rainbow Dash, and Guile, the Sonic Boom man himself. Other captions from the game have also been used as starting prompts to other games, just one more testament to how engrained the original is in Drawception culture.