The Annual Hydralisk Derby was a terran hunting competition that took place in the wastelands of Mar Sara after it was re-colonized in 2502. It involved hunting feral hydralisks that had arrived after the protoss' purification of Mar Sara. The second one was held shortly before the Second Great War. 
In the original artwork for the Annual Hydralisk Derby, Tychus Findlay could be seen in the background, but was replaced by a regular marine in the final art. The hydralisk derby's number was also changed from the 4th annual to the 2nd to match with Mar Sara's recolonization.