It was intended that the hydralisk make a return in StarCraft II from the outset. Some visual upgrades had to be carried out however—the in-game artwork of the hydralisk from the original game did not match the cinematic version, so adjustments were made to the StarCraft II hydralisk such as the tail spikes and head-gills used during the attack animations. Another visual update was the scything talons being increased from one to three to make the design dissimilar from other popular alien designs such as the tyranids.
The hydralisk was also iterated on several times. Earlier renditions were very accurate to Samwise Didier's concept and worked very well in-game. The cinematic department created a high-resolution version of the hydralisk which featured many stylized differences which became a visual disconnect between the cinematic and in-game artwork of the hydralisk. The final iteration of the hydralisk was done with the intention of applying more cinematic-influenced designs to the low-resolution hydralisk while maintaining the bold Samwise proportions and style.
Hydralisks have constantly moved in tier from 1.5 to 2 and back over the development of StarCraft II. In April 2009 the hydralisk has been moved back to tier 1.5, as part of an effort to prevent zerg vs zerg matches devolving into roach vs roach battles. In May 2009 they were moved back up to tier 2. As of June 2009 they were moved back to tier 1.5.
Hydralisks used to deal extra damage to air units.