Drawn for the EqD Training Ground - Wonderbolts Academy Edition
 
I went for Tier 3, which was to:  
-draw a non pegasus celebrating being invited into Wonderbolts selection  
-draw them flying  
-draw their uniform.
 
The Earth Pony is named Wing Nut, and the ill Pegasus is his older brother, Slipstream.  
The tl;dr of the backstory I accidentally came up with is that Slip dreamed of trying out for the Wonderbolts, but became too ill to fly. His younger brother, Wing Nut, who has looked up to him all his life, built a flying machine and went on in his brother’s place to make him proud.